linux/Documentation/memory-barriers.txt
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   1                         ============================
   2                         LINUX KERNEL MEMORY BARRIERS
   3                         ============================
   4
   5By: David Howells <dhowells@redhat.com>
   6    Paul E. McKenney <paulmck@linux.ibm.com>
   7    Will Deacon <will.deacon@arm.com>
   8    Peter Zijlstra <peterz@infradead.org>
   9
  10==========
  11DISCLAIMER
  12==========
  13
  14This document is not a specification; it is intentionally (for the sake of
  15brevity) and unintentionally (due to being human) incomplete. This document is
  16meant as a guide to using the various memory barriers provided by Linux, but
  17in case of any doubt (and there are many) please ask.  Some doubts may be
  18resolved by referring to the formal memory consistency model and related
  19documentation at tools/memory-model/.  Nevertheless, even this memory
  20model should be viewed as the collective opinion of its maintainers rather
  21than as an infallible oracle.
  22
  23To repeat, this document is not a specification of what Linux expects from
  24hardware.
  25
  26The purpose of this document is twofold:
  27
  28 (1) to specify the minimum functionality that one can rely on for any
  29     particular barrier, and
  30
  31 (2) to provide a guide as to how to use the barriers that are available.
  32
  33Note that an architecture can provide more than the minimum requirement
  34for any particular barrier, but if the architecture provides less than
  35that, that architecture is incorrect.
  36
  37Note also that it is possible that a barrier may be a no-op for an
  38architecture because the way that arch works renders an explicit barrier
  39unnecessary in that case.
  40
  41
  42========
  43CONTENTS
  44========
  45
  46 (*) Abstract memory access model.
  47
  48     - Device operations.
  49     - Guarantees.
  50
  51 (*) What are memory barriers?
  52
  53     - Varieties of memory barrier.
  54     - What may not be assumed about memory barriers?
  55     - Data dependency barriers (historical).
  56     - Control dependencies.
  57     - SMP barrier pairing.
  58     - Examples of memory barrier sequences.
  59     - Read memory barriers vs load speculation.
  60     - Multicopy atomicity.
  61
  62 (*) Explicit kernel barriers.
  63
  64     - Compiler barrier.
  65     - CPU memory barriers.
  66
  67 (*) Implicit kernel memory barriers.
  68
  69     - Lock acquisition functions.
  70     - Interrupt disabling functions.
  71     - Sleep and wake-up functions.
  72     - Miscellaneous functions.
  73
  74 (*) Inter-CPU acquiring barrier effects.
  75
  76     - Acquires vs memory accesses.
  77
  78 (*) Where are memory barriers needed?
  79
  80     - Interprocessor interaction.
  81     - Atomic operations.
  82     - Accessing devices.
  83     - Interrupts.
  84
  85 (*) Kernel I/O barrier effects.
  86
  87 (*) Assumed minimum execution ordering model.
  88
  89 (*) The effects of the cpu cache.
  90
  91     - Cache coherency.
  92     - Cache coherency vs DMA.
  93     - Cache coherency vs MMIO.
  94
  95 (*) The things CPUs get up to.
  96
  97     - And then there's the Alpha.
  98     - Virtual Machine Guests.
  99
 100 (*) Example uses.
 101
 102     - Circular buffers.
 103
 104 (*) References.
 105
 106
 107============================
 108ABSTRACT MEMORY ACCESS MODEL
 109============================
 110
 111Consider the following abstract model of the system:
 112
 113                            :                :
 114                            :                :
 115                            :                :
 116                +-------+   :   +--------+   :   +-------+
 117                |       |   :   |        |   :   |       |
 118                |       |   :   |        |   :   |       |
 119                | CPU 1 |<----->| Memory |<----->| CPU 2 |
 120                |       |   :   |        |   :   |       |
 121                |       |   :   |        |   :   |       |
 122                +-------+   :   +--------+   :   +-------+
 123                    ^       :       ^        :       ^
 124                    |       :       |        :       |
 125                    |       :       |        :       |
 126                    |       :       v        :       |
 127                    |       :   +--------+   :       |
 128                    |       :   |        |   :       |
 129                    |       :   |        |   :       |
 130                    +---------->| Device |<----------+
 131                            :   |        |   :
 132                            :   |        |   :
 133                            :   +--------+   :
 134                            :                :
 135
 136Each CPU executes a program that generates memory access operations.  In the
 137abstract CPU, memory operation ordering is very relaxed, and a CPU may actually
 138perform the memory operations in any order it likes, provided program causality
 139appears to be maintained.  Similarly, the compiler may also arrange the
 140instructions it emits in any order it likes, provided it doesn't affect the
 141apparent operation of the program.
 142
 143So in the above diagram, the effects of the memory operations performed by a
 144CPU are perceived by the rest of the system as the operations cross the
 145interface between the CPU and rest of the system (the dotted lines).
 146
 147
 148For example, consider the following sequence of events:
 149
 150        CPU 1           CPU 2
 151        =============== ===============
 152        { A == 1; B == 2 }
 153        A = 3;          x = B;
 154        B = 4;          y = A;
 155
 156The set of accesses as seen by the memory system in the middle can be arranged
 157in 24 different combinations:
 158
 159        STORE A=3,      STORE B=4,      y=LOAD A->3,    x=LOAD B->4
 160        STORE A=3,      STORE B=4,      x=LOAD B->4,    y=LOAD A->3
 161        STORE A=3,      y=LOAD A->3,    STORE B=4,      x=LOAD B->4
 162        STORE A=3,      y=LOAD A->3,    x=LOAD B->2,    STORE B=4
 163        STORE A=3,      x=LOAD B->2,    STORE B=4,      y=LOAD A->3
 164        STORE A=3,      x=LOAD B->2,    y=LOAD A->3,    STORE B=4
 165        STORE B=4,      STORE A=3,      y=LOAD A->3,    x=LOAD B->4
 166        STORE B=4, ...
 167        ...
 168
 169and can thus result in four different combinations of values:
 170
 171        x == 2, y == 1
 172        x == 2, y == 3
 173        x == 4, y == 1
 174        x == 4, y == 3
 175
 176
 177Furthermore, the stores committed by a CPU to the memory system may not be
 178perceived by the loads made by another CPU in the same order as the stores were
 179committed.
 180
 181
 182As a further example, consider this sequence of events:
 183
 184        CPU 1           CPU 2
 185        =============== ===============
 186        { A == 1, B == 2, C == 3, P == &A, Q == &C }
 187        B = 4;          Q = P;
 188        P = &B;         D = *Q;
 189
 190There is an obvious data dependency here, as the value loaded into D depends on
 191the address retrieved from P by CPU 2.  At the end of the sequence, any of the
 192following results are possible:
 193
 194        (Q == &A) and (D == 1)
 195        (Q == &B) and (D == 2)
 196        (Q == &B) and (D == 4)
 197
 198Note that CPU 2 will never try and load C into D because the CPU will load P
 199into Q before issuing the load of *Q.
 200
 201
 202DEVICE OPERATIONS
 203-----------------
 204
 205Some devices present their control interfaces as collections of memory
 206locations, but the order in which the control registers are accessed is very
 207important.  For instance, imagine an ethernet card with a set of internal
 208registers that are accessed through an address port register (A) and a data
 209port register (D).  To read internal register 5, the following code might then
 210be used:
 211
 212        *A = 5;
 213        x = *D;
 214
 215but this might show up as either of the following two sequences:
 216
 217        STORE *A = 5, x = LOAD *D
 218        x = LOAD *D, STORE *A = 5
 219
 220the second of which will almost certainly result in a malfunction, since it set
 221the address _after_ attempting to read the register.
 222
 223
 224GUARANTEES
 225----------
 226
 227There are some minimal guarantees that may be expected of a CPU:
 228
 229 (*) On any given CPU, dependent memory accesses will be issued in order, with
 230     respect to itself.  This means that for:
 231
 232        Q = READ_ONCE(P); D = READ_ONCE(*Q);
 233
 234     the CPU will issue the following memory operations:
 235
 236        Q = LOAD P, D = LOAD *Q
 237
 238     and always in that order.  However, on DEC Alpha, READ_ONCE() also
 239     emits a memory-barrier instruction, so that a DEC Alpha CPU will
 240     instead issue the following memory operations:
 241
 242        Q = LOAD P, MEMORY_BARRIER, D = LOAD *Q, MEMORY_BARRIER
 243
 244     Whether on DEC Alpha or not, the READ_ONCE() also prevents compiler
 245     mischief.
 246
 247 (*) Overlapping loads and stores within a particular CPU will appear to be
 248     ordered within that CPU.  This means that for:
 249
 250        a = READ_ONCE(*X); WRITE_ONCE(*X, b);
 251
 252     the CPU will only issue the following sequence of memory operations:
 253
 254        a = LOAD *X, STORE *X = b
 255
 256     And for:
 257
 258        WRITE_ONCE(*X, c); d = READ_ONCE(*X);
 259
 260     the CPU will only issue:
 261
 262        STORE *X = c, d = LOAD *X
 263
 264     (Loads and stores overlap if they are targeted at overlapping pieces of
 265     memory).
 266
 267And there are a number of things that _must_ or _must_not_ be assumed:
 268
 269 (*) It _must_not_ be assumed that the compiler will do what you want
 270     with memory references that are not protected by READ_ONCE() and
 271     WRITE_ONCE().  Without them, the compiler is within its rights to
 272     do all sorts of "creative" transformations, which are covered in
 273     the COMPILER BARRIER section.
 274
 275 (*) It _must_not_ be assumed that independent loads and stores will be issued
 276     in the order given.  This means that for:
 277
 278        X = *A; Y = *B; *D = Z;
 279
 280     we may get any of the following sequences:
 281
 282        X = LOAD *A,  Y = LOAD *B,  STORE *D = Z
 283        X = LOAD *A,  STORE *D = Z, Y = LOAD *B
 284        Y = LOAD *B,  X = LOAD *A,  STORE *D = Z
 285        Y = LOAD *B,  STORE *D = Z, X = LOAD *A
 286        STORE *D = Z, X = LOAD *A,  Y = LOAD *B
 287        STORE *D = Z, Y = LOAD *B,  X = LOAD *A
 288
 289 (*) It _must_ be assumed that overlapping memory accesses may be merged or
 290     discarded.  This means that for:
 291
 292        X = *A; Y = *(A + 4);
 293
 294     we may get any one of the following sequences:
 295
 296        X = LOAD *A; Y = LOAD *(A + 4);
 297        Y = LOAD *(A + 4); X = LOAD *A;
 298        {X, Y} = LOAD {*A, *(A + 4) };
 299
 300     And for:
 301
 302        *A = X; *(A + 4) = Y;
 303
 304     we may get any of:
 305
 306        STORE *A = X; STORE *(A + 4) = Y;
 307        STORE *(A + 4) = Y; STORE *A = X;
 308        STORE {*A, *(A + 4) } = {X, Y};
 309
 310And there are anti-guarantees:
 311
 312 (*) These guarantees do not apply to bitfields, because compilers often
 313     generate code to modify these using non-atomic read-modify-write
 314     sequences.  Do not attempt to use bitfields to synchronize parallel
 315     algorithms.
 316
 317 (*) Even in cases where bitfields are protected by locks, all fields
 318     in a given bitfield must be protected by one lock.  If two fields
 319     in a given bitfield are protected by different locks, the compiler's
 320     non-atomic read-modify-write sequences can cause an update to one
 321     field to corrupt the value of an adjacent field.
 322
 323 (*) These guarantees apply only to properly aligned and sized scalar
 324     variables.  "Properly sized" currently means variables that are
 325     the same size as "char", "short", "int" and "long".  "Properly
 326     aligned" means the natural alignment, thus no constraints for
 327     "char", two-byte alignment for "short", four-byte alignment for
 328     "int", and either four-byte or eight-byte alignment for "long",
 329     on 32-bit and 64-bit systems, respectively.  Note that these
 330     guarantees were introduced into the C11 standard, so beware when
 331     using older pre-C11 compilers (for example, gcc 4.6).  The portion
 332     of the standard containing this guarantee is Section 3.14, which
 333     defines "memory location" as follows:
 334
 335        memory location
 336                either an object of scalar type, or a maximal sequence
 337                of adjacent bit-fields all having nonzero width
 338
 339                NOTE 1: Two threads of execution can update and access
 340                separate memory locations without interfering with
 341                each other.
 342
 343                NOTE 2: A bit-field and an adjacent non-bit-field member
 344                are in separate memory locations. The same applies
 345                to two bit-fields, if one is declared inside a nested
 346                structure declaration and the other is not, or if the two
 347                are separated by a zero-length bit-field declaration,
 348                or if they are separated by a non-bit-field member
 349                declaration. It is not safe to concurrently update two
 350                bit-fields in the same structure if all members declared
 351                between them are also bit-fields, no matter what the
 352                sizes of those intervening bit-fields happen to be.
 353
 354
 355=========================
 356WHAT ARE MEMORY BARRIERS?
 357=========================
 358
 359As can be seen above, independent memory operations are effectively performed
 360in random order, but this can be a problem for CPU-CPU interaction and for I/O.
 361What is required is some way of intervening to instruct the compiler and the
 362CPU to restrict the order.
 363
 364Memory barriers are such interventions.  They impose a perceived partial
 365ordering over the memory operations on either side of the barrier.
 366
 367Such enforcement is important because the CPUs and other devices in a system
 368can use a variety of tricks to improve performance, including reordering,
 369deferral and combination of memory operations; speculative loads; speculative
 370branch prediction and various types of caching.  Memory barriers are used to
 371override or suppress these tricks, allowing the code to sanely control the
 372interaction of multiple CPUs and/or devices.
 373
 374
 375VARIETIES OF MEMORY BARRIER
 376---------------------------
 377
 378Memory barriers come in four basic varieties:
 379
 380 (1) Write (or store) memory barriers.
 381
 382     A write memory barrier gives a guarantee that all the STORE operations
 383     specified before the barrier will appear to happen before all the STORE
 384     operations specified after the barrier with respect to the other
 385     components of the system.
 386
 387     A write barrier is a partial ordering on stores only; it is not required
 388     to have any effect on loads.
 389
 390     A CPU can be viewed as committing a sequence of store operations to the
 391     memory system as time progresses.  All stores _before_ a write barrier
 392     will occur _before_ all the stores after the write barrier.
 393
 394     [!] Note that write barriers should normally be paired with read or data
 395     dependency barriers; see the "SMP barrier pairing" subsection.
 396
 397
 398 (2) Data dependency barriers.
 399
 400     A data dependency barrier is a weaker form of read barrier.  In the case
 401     where two loads are performed such that the second depends on the result
 402     of the first (eg: the first load retrieves the address to which the second
 403     load will be directed), a data dependency barrier would be required to
 404     make sure that the target of the second load is updated after the address
 405     obtained by the first load is accessed.
 406
 407     A data dependency barrier is a partial ordering on interdependent loads
 408     only; it is not required to have any effect on stores, independent loads
 409     or overlapping loads.
 410
 411     As mentioned in (1), the other CPUs in the system can be viewed as
 412     committing sequences of stores to the memory system that the CPU being
 413     considered can then perceive.  A data dependency barrier issued by the CPU
 414     under consideration guarantees that for any load preceding it, if that
 415     load touches one of a sequence of stores from another CPU, then by the
 416     time the barrier completes, the effects of all the stores prior to that
 417     touched by the load will be perceptible to any loads issued after the data
 418     dependency barrier.
 419
 420     See the "Examples of memory barrier sequences" subsection for diagrams
 421     showing the ordering constraints.
 422
 423     [!] Note that the first load really has to have a _data_ dependency and
 424     not a control dependency.  If the address for the second load is dependent
 425     on the first load, but the dependency is through a conditional rather than
 426     actually loading the address itself, then it's a _control_ dependency and
 427     a full read barrier or better is required.  See the "Control dependencies"
 428     subsection for more information.
 429
 430     [!] Note that data dependency barriers should normally be paired with
 431     write barriers; see the "SMP barrier pairing" subsection.
 432
 433
 434 (3) Read (or load) memory barriers.
 435
 436     A read barrier is a data dependency barrier plus a guarantee that all the
 437     LOAD operations specified before the barrier will appear to happen before
 438     all the LOAD operations specified after the barrier with respect to the
 439     other components of the system.
 440
 441     A read barrier is a partial ordering on loads only; it is not required to
 442     have any effect on stores.
 443
 444     Read memory barriers imply data dependency barriers, and so can substitute
 445     for them.
 446
 447     [!] Note that read barriers should normally be paired with write barriers;
 448     see the "SMP barrier pairing" subsection.
 449
 450
 451 (4) General memory barriers.
 452
 453     A general memory barrier gives a guarantee that all the LOAD and STORE
 454     operations specified before the barrier will appear to happen before all
 455     the LOAD and STORE operations specified after the barrier with respect to
 456     the other components of the system.
 457
 458     A general memory barrier is a partial ordering over both loads and stores.
 459
 460     General memory barriers imply both read and write memory barriers, and so
 461     can substitute for either.
 462
 463
 464And a couple of implicit varieties:
 465
 466 (5) ACQUIRE operations.
 467
 468     This acts as a one-way permeable barrier.  It guarantees that all memory
 469     operations after the ACQUIRE operation will appear to happen after the
 470     ACQUIRE operation with respect to the other components of the system.
 471     ACQUIRE operations include LOCK operations and both smp_load_acquire()
 472     and smp_cond_load_acquire() operations.
 473
 474     Memory operations that occur before an ACQUIRE operation may appear to
 475     happen after it completes.
 476
 477     An ACQUIRE operation should almost always be paired with a RELEASE
 478     operation.
 479
 480
 481 (6) RELEASE operations.
 482
 483     This also acts as a one-way permeable barrier.  It guarantees that all
 484     memory operations before the RELEASE operation will appear to happen
 485     before the RELEASE operation with respect to the other components of the
 486     system. RELEASE operations include UNLOCK operations and
 487     smp_store_release() operations.
 488
 489     Memory operations that occur after a RELEASE operation may appear to
 490     happen before it completes.
 491
 492     The use of ACQUIRE and RELEASE operations generally precludes the need
 493     for other sorts of memory barrier.  In addition, a RELEASE+ACQUIRE pair is
 494     -not- guaranteed to act as a full memory barrier.  However, after an
 495     ACQUIRE on a given variable, all memory accesses preceding any prior
 496     RELEASE on that same variable are guaranteed to be visible.  In other
 497     words, within a given variable's critical section, all accesses of all
 498     previous critical sections for that variable are guaranteed to have
 499     completed.
 500
 501     This means that ACQUIRE acts as a minimal "acquire" operation and
 502     RELEASE acts as a minimal "release" operation.
 503
 504A subset of the atomic operations described in atomic_t.txt have ACQUIRE and
 505RELEASE variants in addition to fully-ordered and relaxed (no barrier
 506semantics) definitions.  For compound atomics performing both a load and a
 507store, ACQUIRE semantics apply only to the load and RELEASE semantics apply
 508only to the store portion of the operation.
 509
 510Memory barriers are only required where there's a possibility of interaction
 511between two CPUs or between a CPU and a device.  If it can be guaranteed that
 512there won't be any such interaction in any particular piece of code, then
 513memory barriers are unnecessary in that piece of code.
 514
 515
 516Note that these are the _minimum_ guarantees.  Different architectures may give
 517more substantial guarantees, but they may _not_ be relied upon outside of arch
 518specific code.
 519
 520
 521WHAT MAY NOT BE ASSUMED ABOUT MEMORY BARRIERS?
 522----------------------------------------------
 523
 524There are certain things that the Linux kernel memory barriers do not guarantee:
 525
 526 (*) There is no guarantee that any of the memory accesses specified before a
 527     memory barrier will be _complete_ by the completion of a memory barrier
 528     instruction; the barrier can be considered to draw a line in that CPU's
 529     access queue that accesses of the appropriate type may not cross.
 530
 531 (*) There is no guarantee that issuing a memory barrier on one CPU will have
 532     any direct effect on another CPU or any other hardware in the system.  The
 533     indirect effect will be the order in which the second CPU sees the effects
 534     of the first CPU's accesses occur, but see the next point:
 535
 536 (*) There is no guarantee that a CPU will see the correct order of effects
 537     from a second CPU's accesses, even _if_ the second CPU uses a memory
 538     barrier, unless the first CPU _also_ uses a matching memory barrier (see
 539     the subsection on "SMP Barrier Pairing").
 540
 541 (*) There is no guarantee that some intervening piece of off-the-CPU
 542     hardware[*] will not reorder the memory accesses.  CPU cache coherency
 543     mechanisms should propagate the indirect effects of a memory barrier
 544     between CPUs, but might not do so in order.
 545
 546        [*] For information on bus mastering DMA and coherency please read:
 547
 548            Documentation/driver-api/pci/pci.rst
 549            Documentation/core-api/dma-api-howto.rst
 550            Documentation/core-api/dma-api.rst
 551
 552
 553DATA DEPENDENCY BARRIERS (HISTORICAL)
 554-------------------------------------
 555
 556As of v4.15 of the Linux kernel, an smp_mb() was added to READ_ONCE() for
 557DEC Alpha, which means that about the only people who need to pay attention
 558to this section are those working on DEC Alpha architecture-specific code
 559and those working on READ_ONCE() itself.  For those who need it, and for
 560those who are interested in the history, here is the story of
 561data-dependency barriers.
 562
 563The usage requirements of data dependency barriers are a little subtle, and
 564it's not always obvious that they're needed.  To illustrate, consider the
 565following sequence of events:
 566
 567        CPU 1                 CPU 2
 568        ===============       ===============
 569        { A == 1, B == 2, C == 3, P == &A, Q == &C }
 570        B = 4;
 571        <write barrier>
 572        WRITE_ONCE(P, &B);
 573                              Q = READ_ONCE(P);
 574                              D = *Q;
 575
 576There's a clear data dependency here, and it would seem that by the end of the
 577sequence, Q must be either &A or &B, and that:
 578
 579        (Q == &A) implies (D == 1)
 580        (Q == &B) implies (D == 4)
 581
 582But!  CPU 2's perception of P may be updated _before_ its perception of B, thus
 583leading to the following situation:
 584
 585        (Q == &B) and (D == 2) ????
 586
 587While this may seem like a failure of coherency or causality maintenance, it
 588isn't, and this behaviour can be observed on certain real CPUs (such as the DEC
 589Alpha).
 590
 591To deal with this, a data dependency barrier or better must be inserted
 592between the address load and the data load:
 593
 594        CPU 1                 CPU 2
 595        ===============       ===============
 596        { A == 1, B == 2, C == 3, P == &A, Q == &C }
 597        B = 4;
 598        <write barrier>
 599        WRITE_ONCE(P, &B);
 600                              Q = READ_ONCE(P);
 601                              <data dependency barrier>
 602                              D = *Q;
 603
 604This enforces the occurrence of one of the two implications, and prevents the
 605third possibility from arising.
 606
 607
 608[!] Note that this extremely counterintuitive situation arises most easily on
 609machines with split caches, so that, for example, one cache bank processes
 610even-numbered cache lines and the other bank processes odd-numbered cache
 611lines.  The pointer P might be stored in an odd-numbered cache line, and the
 612variable B might be stored in an even-numbered cache line.  Then, if the
 613even-numbered bank of the reading CPU's cache is extremely busy while the
 614odd-numbered bank is idle, one can see the new value of the pointer P (&B),
 615but the old value of the variable B (2).
 616
 617
 618A data-dependency barrier is not required to order dependent writes
 619because the CPUs that the Linux kernel supports don't do writes
 620until they are certain (1) that the write will actually happen, (2)
 621of the location of the write, and (3) of the value to be written.
 622But please carefully read the "CONTROL DEPENDENCIES" section and the
 623Documentation/RCU/rcu_dereference.rst file:  The compiler can and does
 624break dependencies in a great many highly creative ways.
 625
 626        CPU 1                 CPU 2
 627        ===============       ===============
 628        { A == 1, B == 2, C = 3, P == &A, Q == &C }
 629        B = 4;
 630        <write barrier>
 631        WRITE_ONCE(P, &B);
 632                              Q = READ_ONCE(P);
 633                              WRITE_ONCE(*Q, 5);
 634
 635Therefore, no data-dependency barrier is required to order the read into
 636Q with the store into *Q.  In other words, this outcome is prohibited,
 637even without a data-dependency barrier:
 638
 639        (Q == &B) && (B == 4)
 640
 641Please note that this pattern should be rare.  After all, the whole point
 642of dependency ordering is to -prevent- writes to the data structure, along
 643with the expensive cache misses associated with those writes.  This pattern
 644can be used to record rare error conditions and the like, and the CPUs'
 645naturally occurring ordering prevents such records from being lost.
 646
 647
 648Note well that the ordering provided by a data dependency is local to
 649the CPU containing it.  See the section on "Multicopy atomicity" for
 650more information.
 651
 652
 653The data dependency barrier is very important to the RCU system,
 654for example.  See rcu_assign_pointer() and rcu_dereference() in
 655include/linux/rcupdate.h.  This permits the current target of an RCU'd
 656pointer to be replaced with a new modified target, without the replacement
 657target appearing to be incompletely initialised.
 658
 659See also the subsection on "Cache Coherency" for a more thorough example.
 660
 661
 662CONTROL DEPENDENCIES
 663--------------------
 664
 665Control dependencies can be a bit tricky because current compilers do
 666not understand them.  The purpose of this section is to help you prevent
 667the compiler's ignorance from breaking your code.
 668
 669A load-load control dependency requires a full read memory barrier, not
 670simply a data dependency barrier to make it work correctly.  Consider the
 671following bit of code:
 672
 673        q = READ_ONCE(a);
 674        if (q) {
 675                <data dependency barrier>  /* BUG: No data dependency!!! */
 676                p = READ_ONCE(b);
 677        }
 678
 679This will not have the desired effect because there is no actual data
 680dependency, but rather a control dependency that the CPU may short-circuit
 681by attempting to predict the outcome in advance, so that other CPUs see
 682the load from b as having happened before the load from a.  In such a
 683case what's actually required is:
 684
 685        q = READ_ONCE(a);
 686        if (q) {
 687                <read barrier>
 688                p = READ_ONCE(b);
 689        }
 690
 691However, stores are not speculated.  This means that ordering -is- provided
 692for load-store control dependencies, as in the following example:
 693
 694        q = READ_ONCE(a);
 695        if (q) {
 696                WRITE_ONCE(b, 1);
 697        }
 698
 699Control dependencies pair normally with other types of barriers.
 700That said, please note that neither READ_ONCE() nor WRITE_ONCE()
 701are optional! Without the READ_ONCE(), the compiler might combine the
 702load from 'a' with other loads from 'a'.  Without the WRITE_ONCE(),
 703the compiler might combine the store to 'b' with other stores to 'b'.
 704Either can result in highly counterintuitive effects on ordering.
 705
 706Worse yet, if the compiler is able to prove (say) that the value of
 707variable 'a' is always non-zero, it would be well within its rights
 708to optimize the original example by eliminating the "if" statement
 709as follows:
 710
 711        q = a;
 712        b = 1;  /* BUG: Compiler and CPU can both reorder!!! */
 713
 714So don't leave out the READ_ONCE().
 715
 716It is tempting to try to enforce ordering on identical stores on both
 717branches of the "if" statement as follows:
 718
 719        q = READ_ONCE(a);
 720        if (q) {
 721                barrier();
 722                WRITE_ONCE(b, 1);
 723                do_something();
 724        } else {
 725                barrier();
 726                WRITE_ONCE(b, 1);
 727                do_something_else();
 728        }
 729
 730Unfortunately, current compilers will transform this as follows at high
 731optimization levels:
 732
 733        q = READ_ONCE(a);
 734        barrier();
 735        WRITE_ONCE(b, 1);  /* BUG: No ordering vs. load from a!!! */
 736        if (q) {
 737                /* WRITE_ONCE(b, 1); -- moved up, BUG!!! */
 738                do_something();
 739        } else {
 740                /* WRITE_ONCE(b, 1); -- moved up, BUG!!! */
 741                do_something_else();
 742        }
 743
 744Now there is no conditional between the load from 'a' and the store to
 745'b', which means that the CPU is within its rights to reorder them:
 746The conditional is absolutely required, and must be present in the
 747assembly code even after all compiler optimizations have been applied.
 748Therefore, if you need ordering in this example, you need explicit
 749memory barriers, for example, smp_store_release():
 750
 751        q = READ_ONCE(a);
 752        if (q) {
 753                smp_store_release(&b, 1);
 754                do_something();
 755        } else {
 756                smp_store_release(&b, 1);
 757                do_something_else();
 758        }
 759
 760In contrast, without explicit memory barriers, two-legged-if control
 761ordering is guaranteed only when the stores differ, for example:
 762
 763        q = READ_ONCE(a);
 764        if (q) {
 765                WRITE_ONCE(b, 1);
 766                do_something();
 767        } else {
 768                WRITE_ONCE(b, 2);
 769                do_something_else();
 770        }
 771
 772The initial READ_ONCE() is still required to prevent the compiler from
 773proving the value of 'a'.
 774
 775In addition, you need to be careful what you do with the local variable 'q',
 776otherwise the compiler might be able to guess the value and again remove
 777the needed conditional.  For example:
 778
 779        q = READ_ONCE(a);
 780        if (q % MAX) {
 781                WRITE_ONCE(b, 1);
 782                do_something();
 783        } else {
 784                WRITE_ONCE(b, 2);
 785                do_something_else();
 786        }
 787
 788If MAX is defined to be 1, then the compiler knows that (q % MAX) is
 789equal to zero, in which case the compiler is within its rights to
 790transform the above code into the following:
 791
 792        q = READ_ONCE(a);
 793        WRITE_ONCE(b, 2);
 794        do_something_else();
 795
 796Given this transformation, the CPU is not required to respect the ordering
 797between the load from variable 'a' and the store to variable 'b'.  It is
 798tempting to add a barrier(), but this does not help.  The conditional
 799is gone, and the barrier won't bring it back.  Therefore, if you are
 800relying on this ordering, you should make sure that MAX is greater than
 801one, perhaps as follows:
 802
 803        q = READ_ONCE(a);
 804        BUILD_BUG_ON(MAX <= 1); /* Order load from a with store to b. */
 805        if (q % MAX) {
 806                WRITE_ONCE(b, 1);
 807                do_something();
 808        } else {
 809                WRITE_ONCE(b, 2);
 810                do_something_else();
 811        }
 812
 813Please note once again that the stores to 'b' differ.  If they were
 814identical, as noted earlier, the compiler could pull this store outside
 815of the 'if' statement.
 816
 817You must also be careful not to rely too much on boolean short-circuit
 818evaluation.  Consider this example:
 819
 820        q = READ_ONCE(a);
 821        if (q || 1 > 0)
 822                WRITE_ONCE(b, 1);
 823
 824Because the first condition cannot fault and the second condition is
 825always true, the compiler can transform this example as following,
 826defeating control dependency:
 827
 828        q = READ_ONCE(a);
 829        WRITE_ONCE(b, 1);
 830
 831This example underscores the need to ensure that the compiler cannot
 832out-guess your code.  More generally, although READ_ONCE() does force
 833the compiler to actually emit code for a given load, it does not force
 834the compiler to use the results.
 835
 836In addition, control dependencies apply only to the then-clause and
 837else-clause of the if-statement in question.  In particular, it does
 838not necessarily apply to code following the if-statement:
 839
 840        q = READ_ONCE(a);
 841        if (q) {
 842                WRITE_ONCE(b, 1);
 843        } else {
 844                WRITE_ONCE(b, 2);
 845        }
 846        WRITE_ONCE(c, 1);  /* BUG: No ordering against the read from 'a'. */
 847
 848It is tempting to argue that there in fact is ordering because the
 849compiler cannot reorder volatile accesses and also cannot reorder
 850the writes to 'b' with the condition.  Unfortunately for this line
 851of reasoning, the compiler might compile the two writes to 'b' as
 852conditional-move instructions, as in this fanciful pseudo-assembly
 853language:
 854
 855        ld r1,a
 856        cmp r1,$0
 857        cmov,ne r4,$1
 858        cmov,eq r4,$2
 859        st r4,b
 860        st $1,c
 861
 862A weakly ordered CPU would have no dependency of any sort between the load
 863from 'a' and the store to 'c'.  The control dependencies would extend
 864only to the pair of cmov instructions and the store depending on them.
 865In short, control dependencies apply only to the stores in the then-clause
 866and else-clause of the if-statement in question (including functions
 867invoked by those two clauses), not to code following that if-statement.
 868
 869
 870Note well that the ordering provided by a control dependency is local
 871to the CPU containing it.  See the section on "Multicopy atomicity"
 872for more information.
 873
 874
 875In summary:
 876
 877  (*) Control dependencies can order prior loads against later stores.
 878      However, they do -not- guarantee any other sort of ordering:
 879      Not prior loads against later loads, nor prior stores against
 880      later anything.  If you need these other forms of ordering,
 881      use smp_rmb(), smp_wmb(), or, in the case of prior stores and
 882      later loads, smp_mb().
 883
 884  (*) If both legs of the "if" statement begin with identical stores to
 885      the same variable, then those stores must be ordered, either by
 886      preceding both of them with smp_mb() or by using smp_store_release()
 887      to carry out the stores.  Please note that it is -not- sufficient
 888      to use barrier() at beginning of each leg of the "if" statement
 889      because, as shown by the example above, optimizing compilers can
 890      destroy the control dependency while respecting the letter of the
 891      barrier() law.
 892
 893  (*) Control dependencies require at least one run-time conditional
 894      between the prior load and the subsequent store, and this
 895      conditional must involve the prior load.  If the compiler is able
 896      to optimize the conditional away, it will have also optimized
 897      away the ordering.  Careful use of READ_ONCE() and WRITE_ONCE()
 898      can help to preserve the needed conditional.
 899
 900  (*) Control dependencies require that the compiler avoid reordering the
 901      dependency into nonexistence.  Careful use of READ_ONCE() or
 902      atomic{,64}_read() can help to preserve your control dependency.
 903      Please see the COMPILER BARRIER section for more information.
 904
 905  (*) Control dependencies apply only to the then-clause and else-clause
 906      of the if-statement containing the control dependency, including
 907      any functions that these two clauses call.  Control dependencies
 908      do -not- apply to code following the if-statement containing the
 909      control dependency.
 910
 911  (*) Control dependencies pair normally with other types of barriers.
 912
 913  (*) Control dependencies do -not- provide multicopy atomicity.  If you
 914      need all the CPUs to see a given store at the same time, use smp_mb().
 915
 916  (*) Compilers do not understand control dependencies.  It is therefore
 917      your job to ensure that they do not break your code.
 918
 919
 920SMP BARRIER PAIRING
 921-------------------
 922
 923When dealing with CPU-CPU interactions, certain types of memory barrier should
 924always be paired.  A lack of appropriate pairing is almost certainly an error.
 925
 926General barriers pair with each other, though they also pair with most
 927other types of barriers, albeit without multicopy atomicity.  An acquire
 928barrier pairs with a release barrier, but both may also pair with other
 929barriers, including of course general barriers.  A write barrier pairs
 930with a data dependency barrier, a control dependency, an acquire barrier,
 931a release barrier, a read barrier, or a general barrier.  Similarly a
 932read barrier, control dependency, or a data dependency barrier pairs
 933with a write barrier, an acquire barrier, a release barrier, or a
 934general barrier:
 935
 936        CPU 1                 CPU 2
 937        ===============       ===============
 938        WRITE_ONCE(a, 1);
 939        <write barrier>
 940        WRITE_ONCE(b, 2);     x = READ_ONCE(b);
 941                              <read barrier>
 942                              y = READ_ONCE(a);
 943
 944Or:
 945
 946        CPU 1                 CPU 2
 947        ===============       ===============================
 948        a = 1;
 949        <write barrier>
 950        WRITE_ONCE(b, &a);    x = READ_ONCE(b);
 951                              <data dependency barrier>
 952                              y = *x;
 953
 954Or even:
 955
 956        CPU 1                 CPU 2
 957        ===============       ===============================
 958        r1 = READ_ONCE(y);
 959        <general barrier>
 960        WRITE_ONCE(x, 1);     if (r2 = READ_ONCE(x)) {
 961                                 <implicit control dependency>
 962                                 WRITE_ONCE(y, 1);
 963                              }
 964
 965        assert(r1 == 0 || r2 == 0);
 966
 967Basically, the read barrier always has to be there, even though it can be of
 968the "weaker" type.
 969
 970[!] Note that the stores before the write barrier would normally be expected to
 971match the loads after the read barrier or the data dependency barrier, and vice
 972versa:
 973
 974        CPU 1                               CPU 2
 975        ===================                 ===================
 976        WRITE_ONCE(a, 1);    }----   --->{  v = READ_ONCE(c);
 977        WRITE_ONCE(b, 2);    }    \ /    {  w = READ_ONCE(d);
 978        <write barrier>            \        <read barrier>
 979        WRITE_ONCE(c, 3);    }    / \    {  x = READ_ONCE(a);
 980        WRITE_ONCE(d, 4);    }----   --->{  y = READ_ONCE(b);
 981
 982
 983EXAMPLES OF MEMORY BARRIER SEQUENCES
 984------------------------------------
 985
 986Firstly, write barriers act as partial orderings on store operations.
 987Consider the following sequence of events:
 988
 989        CPU 1
 990        =======================
 991        STORE A = 1
 992        STORE B = 2
 993        STORE C = 3
 994        <write barrier>
 995        STORE D = 4
 996        STORE E = 5
 997
 998This sequence of events is committed to the memory coherence system in an order
 999that the rest of the system might perceive as the unordered set of { STORE A,
1000STORE B, STORE C } all occurring before the unordered set of { STORE D, STORE E
1001}:
1002
1003        +-------+       :      :
1004        |       |       +------+
1005        |       |------>| C=3  |     }     /\
1006        |       |  :    +------+     }-----  \  -----> Events perceptible to
1007        |       |  :    | A=1  |     }        \/       the rest of the system
1008        |       |  :    +------+     }
1009        | CPU 1 |  :    | B=2  |     }
1010        |       |       +------+     }
1011        |       |   wwwwwwwwwwwwwwww }   <--- At this point the write barrier
1012        |       |       +------+     }        requires all stores prior to the
1013        |       |  :    | E=5  |     }        barrier to be committed before
1014        |       |  :    +------+     }        further stores may take place
1015        |       |------>| D=4  |     }
1016        |       |       +------+
1017        +-------+       :      :
1018                           |
1019                           | Sequence in which stores are committed to the
1020                           | memory system by CPU 1
1021                           V
1022
1023
1024Secondly, data dependency barriers act as partial orderings on data-dependent
1025loads.  Consider the following sequence of events:
1026
1027        CPU 1                   CPU 2
1028        ======================= =======================
1029                { B = 7; X = 9; Y = 8; C = &Y }
1030        STORE A = 1
1031        STORE B = 2
1032        <write barrier>
1033        STORE C = &B            LOAD X
1034        STORE D = 4             LOAD C (gets &B)
1035                                LOAD *C (reads B)
1036
1037Without intervention, CPU 2 may perceive the events on CPU 1 in some
1038effectively random order, despite the write barrier issued by CPU 1:
1039
1040        +-------+       :      :                :       :
1041        |       |       +------+                +-------+  | Sequence of update
1042        |       |------>| B=2  |-----       --->| Y->8  |  | of perception on
1043        |       |  :    +------+     \          +-------+  | CPU 2
1044        | CPU 1 |  :    | A=1  |      \     --->| C->&Y |  V
1045        |       |       +------+       |        +-------+
1046        |       |   wwwwwwwwwwwwwwww   |        :       :
1047        |       |       +------+       |        :       :
1048        |       |  :    | C=&B |---    |        :       :       +-------+
1049        |       |  :    +------+   \   |        +-------+       |       |
1050        |       |------>| D=4  |    ----------->| C->&B |------>|       |
1051        |       |       +------+       |        +-------+       |       |
1052        +-------+       :      :       |        :       :       |       |
1053                                       |        :       :       |       |
1054                                       |        :       :       | CPU 2 |
1055                                       |        +-------+       |       |
1056            Apparently incorrect --->  |        | B->7  |------>|       |
1057            perception of B (!)        |        +-------+       |       |
1058                                       |        :       :       |       |
1059                                       |        +-------+       |       |
1060            The load of X holds --->    \       | X->9  |------>|       |
1061            up the maintenance           \      +-------+       |       |
1062            of coherence of B             ----->| B->2  |       +-------+
1063                                                +-------+
1064                                                :       :
1065
1066
1067In the above example, CPU 2 perceives that B is 7, despite the load of *C
1068(which would be B) coming after the LOAD of C.
1069
1070If, however, a data dependency barrier were to be placed between the load of C
1071and the load of *C (ie: B) on CPU 2:
1072
1073        CPU 1                   CPU 2
1074        ======================= =======================
1075                { B = 7; X = 9; Y = 8; C = &Y }
1076        STORE A = 1
1077        STORE B = 2
1078        <write barrier>
1079        STORE C = &B            LOAD X
1080        STORE D = 4             LOAD C (gets &B)
1081                                <data dependency barrier>
1082                                LOAD *C (reads B)
1083
1084then the following will occur:
1085
1086        +-------+       :      :                :       :
1087        |       |       +------+                +-------+
1088        |       |------>| B=2  |-----       --->| Y->8  |
1089        |       |  :    +------+     \          +-------+
1090        | CPU 1 |  :    | A=1  |      \     --->| C->&Y |
1091        |       |       +------+       |        +-------+
1092        |       |   wwwwwwwwwwwwwwww   |        :       :
1093        |       |       +------+       |        :       :
1094        |       |  :    | C=&B |---    |        :       :       +-------+
1095        |       |  :    +------+   \   |        +-------+       |       |
1096        |       |------>| D=4  |    ----------->| C->&B |------>|       |
1097        |       |       +------+       |        +-------+       |       |
1098        +-------+       :      :       |        :       :       |       |
1099                                       |        :       :       |       |
1100                                       |        :       :       | CPU 2 |
1101                                       |        +-------+       |       |
1102                                       |        | X->9  |------>|       |
1103                                       |        +-------+       |       |
1104          Makes sure all effects --->   \   ddddddddddddddddd   |       |
1105          prior to the store of C        \      +-------+       |       |
1106          are perceptible to              ----->| B->2  |------>|       |
1107          subsequent loads                      +-------+       |       |
1108                                                :       :       +-------+
1109
1110
1111And thirdly, a read barrier acts as a partial order on loads.  Consider the
1112following sequence of events:
1113
1114        CPU 1                   CPU 2
1115        ======================= =======================
1116                { A = 0, B = 9 }
1117        STORE A=1
1118        <write barrier>
1119        STORE B=2
1120                                LOAD B
1121                                LOAD A
1122
1123Without intervention, CPU 2 may then choose to perceive the events on CPU 1 in
1124some effectively random order, despite the write barrier issued by CPU 1:
1125
1126        +-------+       :      :                :       :
1127        |       |       +------+                +-------+
1128        |       |------>| A=1  |------      --->| A->0  |
1129        |       |       +------+      \         +-------+
1130        | CPU 1 |   wwwwwwwwwwwwwwww   \    --->| B->9  |
1131        |       |       +------+        |       +-------+
1132        |       |------>| B=2  |---     |       :       :
1133        |       |       +------+   \    |       :       :       +-------+
1134        +-------+       :      :    \   |       +-------+       |       |
1135                                     ---------->| B->2  |------>|       |
1136                                        |       +-------+       | CPU 2 |
1137                                        |       | A->0  |------>|       |
1138                                        |       +-------+       |       |
1139                                        |       :       :       +-------+
1140                                         \      :       :
1141                                          \     +-------+
1142                                           ---->| A->1  |
1143                                                +-------+
1144                                                :       :
1145
1146
1147If, however, a read barrier were to be placed between the load of B and the
1148load of A on CPU 2:
1149
1150        CPU 1                   CPU 2
1151        ======================= =======================
1152                { A = 0, B = 9 }
1153        STORE A=1
1154        <write barrier>
1155        STORE B=2
1156                                LOAD B
1157                                <read barrier>
1158                                LOAD A
1159
1160then the partial ordering imposed by CPU 1 will be perceived correctly by CPU
11612:
1162
1163        +-------+       :      :                :       :
1164        |       |       +------+                +-------+
1165        |       |------>| A=1  |------      --->| A->0  |
1166        |       |       +------+      \         +-------+
1167        | CPU 1 |   wwwwwwwwwwwwwwww   \    --->| B->9  |
1168        |       |       +------+        |       +-------+
1169        |       |------>| B=2  |---     |       :       :
1170        |       |       +------+   \    |       :       :       +-------+
1171        +-------+       :      :    \   |       +-------+       |       |
1172                                     ---------->| B->2  |------>|       |
1173                                        |       +-------+       | CPU 2 |
1174                                        |       :       :       |       |
1175                                        |       :       :       |       |
1176          At this point the read ---->   \  rrrrrrrrrrrrrrrrr   |       |
1177          barrier causes all effects      \     +-------+       |       |
1178          prior to the storage of B        ---->| A->1  |------>|       |
1179          to be perceptible to CPU 2            +-------+       |       |
1180                                                :       :       +-------+
1181
1182
1183To illustrate this more completely, consider what could happen if the code
1184contained a load of A either side of the read barrier:
1185
1186        CPU 1                   CPU 2
1187        ======================= =======================
1188                { A = 0, B = 9 }
1189        STORE A=1
1190        <write barrier>
1191        STORE B=2
1192                                LOAD B
1193                                LOAD A [first load of A]
1194                                <read barrier>
1195                                LOAD A [second load of A]
1196
1197Even though the two loads of A both occur after the load of B, they may both
1198come up with different values:
1199
1200        +-------+       :      :                :       :
1201        |       |       +------+                +-------+
1202        |       |------>| A=1  |------      --->| A->0  |
1203        |       |       +------+      \         +-------+
1204        | CPU 1 |   wwwwwwwwwwwwwwww   \    --->| B->9  |
1205        |       |       +------+        |       +-------+
1206        |       |------>| B=2  |---     |       :       :
1207        |       |       +------+   \    |       :       :       +-------+
1208        +-------+       :      :    \   |       +-------+       |       |
1209                                     ---------->| B->2  |------>|       |
1210                                        |       +-------+       | CPU 2 |
1211                                        |       :       :       |       |
1212                                        |       :       :       |       |
1213                                        |       +-------+       |       |
1214                                        |       | A->0  |------>| 1st   |
1215                                        |       +-------+       |       |
1216          At this point the read ---->   \  rrrrrrrrrrrrrrrrr   |       |
1217          barrier causes all effects      \     +-------+       |       |
1218          prior to the storage of B        ---->| A->1  |------>| 2nd   |
1219          to be perceptible to CPU 2            +-------+       |       |
1220                                                :       :       +-------+
1221
1222
1223But it may be that the update to A from CPU 1 becomes perceptible to CPU 2
1224before the read barrier completes anyway:
1225
1226        +-------+       :      :                :       :
1227        |       |       +------+                +-------+
1228        |       |------>| A=1  |------      --->| A->0  |
1229        |       |       +------+      \         +-------+
1230        | CPU 1 |   wwwwwwwwwwwwwwww   \    --->| B->9  |
1231        |       |       +------+        |       +-------+
1232        |       |------>| B=2  |---     |       :       :
1233        |       |       +------+   \    |       :       :       +-------+
1234        +-------+       :      :    \   |       +-------+       |       |
1235                                     ---------->| B->2  |------>|       |
1236                                        |       +-------+       | CPU 2 |
1237                                        |       :       :       |       |
1238                                         \      :       :       |       |
1239                                          \     +-------+       |       |
1240                                           ---->| A->1  |------>| 1st   |
1241                                                +-------+       |       |
1242                                            rrrrrrrrrrrrrrrrr   |       |
1243                                                +-------+       |       |
1244                                                | A->1  |------>| 2nd   |
1245                                                +-------+       |       |
1246                                                :       :       +-------+
1247
1248
1249The guarantee is that the second load will always come up with A == 1 if the
1250load of B came up with B == 2.  No such guarantee exists for the first load of
1251A; that may come up with either A == 0 or A == 1.
1252
1253
1254READ MEMORY BARRIERS VS LOAD SPECULATION
1255----------------------------------------
1256
1257Many CPUs speculate with loads: that is they see that they will need to load an
1258item from memory, and they find a time where they're not using the bus for any
1259other loads, and so do the load in advance - even though they haven't actually
1260got to that point in the instruction execution flow yet.  This permits the
1261actual load instruction to potentially complete immediately because the CPU
1262already has the value to hand.
1263
1264It may turn out that the CPU didn't actually need the value - perhaps because a
1265branch circumvented the load - in which case it can discard the value or just
1266cache it for later use.
1267
1268Consider:
1269
1270        CPU 1                   CPU 2
1271        ======================= =======================
1272                                LOAD B
1273                                DIVIDE          } Divide instructions generally
1274                                DIVIDE          } take a long time to perform
1275                                LOAD A
1276
1277Which might appear as this:
1278
1279                                                :       :       +-------+
1280                                                +-------+       |       |
1281                                            --->| B->2  |------>|       |
1282                                                +-------+       | CPU 2 |
1283                                                :       :DIVIDE |       |
1284                                                +-------+       |       |
1285        The CPU being busy doing a --->     --->| A->0  |~~~~   |       |
1286        division speculates on the              +-------+   ~   |       |
1287        LOAD of A                               :       :   ~   |       |
1288                                                :       :DIVIDE |       |
1289                                                :       :   ~   |       |
1290        Once the divisions are complete -->     :       :   ~-->|       |
1291        the CPU can then perform the            :       :       |       |
1292        LOAD with immediate effect              :       :       +-------+
1293
1294
1295Placing a read barrier or a data dependency barrier just before the second
1296load:
1297
1298        CPU 1                   CPU 2
1299        ======================= =======================
1300                                LOAD B
1301                                DIVIDE
1302                                DIVIDE
1303                                <read barrier>
1304                                LOAD A
1305
1306will force any value speculatively obtained to be reconsidered to an extent
1307dependent on the type of barrier used.  If there was no change made to the
1308speculated memory location, then the speculated value will just be used:
1309
1310                                                :       :       +-------+
1311                                                +-------+       |       |
1312                                            --->| B->2  |------>|       |
1313                                                +-------+       | CPU 2 |
1314                                                :       :DIVIDE |       |
1315                                                +-------+       |       |
1316        The CPU being busy doing a --->     --->| A->0  |~~~~   |       |
1317        division speculates on the              +-------+   ~   |       |
1318        LOAD of A                               :       :   ~   |       |
1319                                                :       :DIVIDE |       |
1320                                                :       :   ~   |       |
1321                                                :       :   ~   |       |
1322                                            rrrrrrrrrrrrrrrr~   |       |
1323                                                :       :   ~   |       |
1324                                                :       :   ~-->|       |
1325                                                :       :       |       |
1326                                                :       :       +-------+
1327
1328
1329but if there was an update or an invalidation from another CPU pending, then
1330the speculation will be cancelled and the value reloaded:
1331
1332                                                :       :       +-------+
1333                                                +-------+       |       |
1334                                            --->| B->2  |------>|       |
1335                                                +-------+       | CPU 2 |
1336                                                :       :DIVIDE |       |
1337                                                +-------+       |       |
1338        The CPU being busy doing a --->     --->| A->0  |~~~~   |       |
1339        division speculates on the              +-------+   ~   |       |
1340        LOAD of A                               :       :   ~   |       |
1341                                                :       :DIVIDE |       |
1342                                                :       :   ~   |       |
1343                                                :       :   ~   |       |
1344                                            rrrrrrrrrrrrrrrrr   |       |
1345                                                +-------+       |       |
1346        The speculation is discarded --->   --->| A->1  |------>|       |
1347        and an updated value is                 +-------+       |       |
1348        retrieved                               :       :       +-------+
1349
1350
1351MULTICOPY ATOMICITY
1352--------------------
1353
1354Multicopy atomicity is a deeply intuitive notion about ordering that is
1355not always provided by real computer systems, namely that a given store
1356becomes visible at the same time to all CPUs, or, alternatively, that all
1357CPUs agree on the order in which all stores become visible.  However,
1358support of full multicopy atomicity would rule out valuable hardware
1359optimizations, so a weaker form called ``other multicopy atomicity''
1360instead guarantees only that a given store becomes visible at the same
1361time to all -other- CPUs.  The remainder of this document discusses this
1362weaker form, but for brevity will call it simply ``multicopy atomicity''.
1363
1364The following example demonstrates multicopy atomicity:
1365
1366        CPU 1                   CPU 2                   CPU 3
1367        ======================= ======================= =======================
1368                { X = 0, Y = 0 }
1369        STORE X=1               r1=LOAD X (reads 1)     LOAD Y (reads 1)
1370                                <general barrier>       <read barrier>
1371                                STORE Y=r1              LOAD X
1372
1373Suppose that CPU 2's load from X returns 1, which it then stores to Y,
1374and CPU 3's load from Y returns 1.  This indicates that CPU 1's store
1375to X precedes CPU 2's load from X and that CPU 2's store to Y precedes
1376CPU 3's load from Y.  In addition, the memory barriers guarantee that
1377CPU 2 executes its load before its store, and CPU 3 loads from Y before
1378it loads from X.  The question is then "Can CPU 3's load from X return 0?"
1379
1380Because CPU 3's load from X in some sense comes after CPU 2's load, it
1381is natural to expect that CPU 3's load from X must therefore return 1.
1382This expectation follows from multicopy atomicity: if a load executing
1383on CPU B follows a load from the same variable executing on CPU A (and
1384CPU A did not originally store the value which it read), then on
1385multicopy-atomic systems, CPU B's load must return either the same value
1386that CPU A's load did or some later value.  However, the Linux kernel
1387does not require systems to be multicopy atomic.
1388
1389The use of a general memory barrier in the example above compensates
1390for any lack of multicopy atomicity.  In the example, if CPU 2's load
1391from X returns 1 and CPU 3's load from Y returns 1, then CPU 3's load
1392from X must indeed also return 1.
1393
1394However, dependencies, read barriers, and write barriers are not always
1395able to compensate for non-multicopy atomicity.  For example, suppose
1396that CPU 2's general barrier is removed from the above example, leaving
1397only the data dependency shown below:
1398
1399        CPU 1                   CPU 2                   CPU 3
1400        ======================= ======================= =======================
1401                { X = 0, Y = 0 }
1402        STORE X=1               r1=LOAD X (reads 1)     LOAD Y (reads 1)
1403                                <data dependency>       <read barrier>
1404                                STORE Y=r1              LOAD X (reads 0)
1405
1406This substitution allows non-multicopy atomicity to run rampant: in
1407this example, it is perfectly legal for CPU 2's load from X to return 1,
1408CPU 3's load from Y to return 1, and its load from X to return 0.
1409
1410The key point is that although CPU 2's data dependency orders its load
1411and store, it does not guarantee to order CPU 1's store.  Thus, if this
1412example runs on a non-multicopy-atomic system where CPUs 1 and 2 share a
1413store buffer or a level of cache, CPU 2 might have early access to CPU 1's
1414writes.  General barriers are therefore required to ensure that all CPUs
1415agree on the combined order of multiple accesses.
1416
1417General barriers can compensate not only for non-multicopy atomicity,
1418but can also generate additional ordering that can ensure that -all-
1419CPUs will perceive the same order of -all- operations.  In contrast, a
1420chain of release-acquire pairs do not provide this additional ordering,
1421which means that only those CPUs on the chain are guaranteed to agree
1422on the combined order of the accesses.  For example, switching to C code
1423in deference to the ghost of Herman Hollerith:
1424
1425        int u, v, x, y, z;
1426
1427        void cpu0(void)
1428        {
1429                r0 = smp_load_acquire(&x);
1430                WRITE_ONCE(u, 1);
1431                smp_store_release(&y, 1);
1432        }
1433
1434        void cpu1(void)
1435        {
1436                r1 = smp_load_acquire(&y);
1437                r4 = READ_ONCE(v);
1438                r5 = READ_ONCE(u);
1439                smp_store_release(&z, 1);
1440        }
1441
1442        void cpu2(void)
1443        {
1444                r2 = smp_load_acquire(&z);
1445                smp_store_release(&x, 1);
1446        }
1447
1448        void cpu3(void)
1449        {
1450                WRITE_ONCE(v, 1);
1451                smp_mb();
1452                r3 = READ_ONCE(u);
1453        }
1454
1455Because cpu0(), cpu1(), and cpu2() participate in a chain of
1456smp_store_release()/smp_load_acquire() pairs, the following outcome
1457is prohibited:
1458
1459        r0 == 1 && r1 == 1 && r2 == 1
1460
1461Furthermore, because of the release-acquire relationship between cpu0()
1462and cpu1(), cpu1() must see cpu0()'s writes, so that the following
1463outcome is prohibited:
1464
1465        r1 == 1 && r5 == 0
1466
1467However, the ordering provided by a release-acquire chain is local
1468to the CPUs participating in that chain and does not apply to cpu3(),
1469at least aside from stores.  Therefore, the following outcome is possible:
1470
1471        r0 == 0 && r1 == 1 && r2 == 1 && r3 == 0 && r4 == 0
1472
1473As an aside, the following outcome is also possible:
1474
1475        r0 == 0 && r1 == 1 && r2 == 1 && r3 == 0 && r4 == 0 && r5 == 1
1476
1477Although cpu0(), cpu1(), and cpu2() will see their respective reads and
1478writes in order, CPUs not involved in the release-acquire chain might
1479well disagree on the order.  This disagreement stems from the fact that
1480the weak memory-barrier instructions used to implement smp_load_acquire()
1481and smp_store_release() are not required to order prior stores against
1482subsequent loads in all cases.  This means that cpu3() can see cpu0()'s
1483store to u as happening -after- cpu1()'s load from v, even though
1484both cpu0() and cpu1() agree that these two operations occurred in the
1485intended order.
1486
1487However, please keep in mind that smp_load_acquire() is not magic.
1488In particular, it simply reads from its argument with ordering.  It does
1489-not- ensure that any particular value will be read.  Therefore, the
1490following outcome is possible:
1491
1492        r0 == 0 && r1 == 0 && r2 == 0 && r5 == 0
1493
1494Note that this outcome can happen even on a mythical sequentially
1495consistent system where nothing is ever reordered.
1496
1497To reiterate, if your code requires full ordering of all operations,
1498use general barriers throughout.
1499
1500
1501========================
1502EXPLICIT KERNEL BARRIERS
1503========================
1504
1505The Linux kernel has a variety of different barriers that act at different
1506levels:
1507
1508  (*) Compiler barrier.
1509
1510  (*) CPU memory barriers.
1511
1512
1513COMPILER BARRIER
1514----------------
1515
1516The Linux kernel has an explicit compiler barrier function that prevents the
1517compiler from moving the memory accesses either side of it to the other side:
1518
1519        barrier();
1520
1521This is a general barrier -- there are no read-read or write-write
1522variants of barrier().  However, READ_ONCE() and WRITE_ONCE() can be
1523thought of as weak forms of barrier() that affect only the specific
1524accesses flagged by the READ_ONCE() or WRITE_ONCE().
1525
1526The barrier() function has the following effects:
1527
1528 (*) Prevents the compiler from reordering accesses following the
1529     barrier() to precede any accesses preceding the barrier().
1530     One example use for this property is to ease communication between
1531     interrupt-handler code and the code that was interrupted.
1532
1533 (*) Within a loop, forces the compiler to load the variables used
1534     in that loop's conditional on each pass through that loop.
1535
1536The READ_ONCE() and WRITE_ONCE() functions can prevent any number of
1537optimizations that, while perfectly safe in single-threaded code, can
1538be fatal in concurrent code.  Here are some examples of these sorts
1539of optimizations:
1540
1541 (*) The compiler is within its rights to reorder loads and stores
1542     to the same variable, and in some cases, the CPU is within its
1543     rights to reorder loads to the same variable.  This means that
1544     the following code:
1545
1546        a[0] = x;
1547        a[1] = x;
1548
1549     Might result in an older value of x stored in a[1] than in a[0].
1550     Prevent both the compiler and the CPU from doing this as follows:
1551
1552        a[0] = READ_ONCE(x);
1553        a[1] = READ_ONCE(x);
1554
1555     In short, READ_ONCE() and WRITE_ONCE() provide cache coherence for
1556     accesses from multiple CPUs to a single variable.
1557
1558 (*) The compiler is within its rights to merge successive loads from
1559     the same variable.  Such merging can cause the compiler to "optimize"
1560     the following code:
1561
1562        while (tmp = a)
1563                do_something_with(tmp);
1564
1565     into the following code, which, although in some sense legitimate
1566     for single-threaded code, is almost certainly not what the developer
1567     intended:
1568
1569        if (tmp = a)
1570                for (;;)
1571                        do_something_with(tmp);
1572
1573     Use READ_ONCE() to prevent the compiler from doing this to you:
1574
1575        while (tmp = READ_ONCE(a))
1576                do_something_with(tmp);
1577
1578 (*) The compiler is within its rights to reload a variable, for example,
1579     in cases where high register pressure prevents the compiler from
1580     keeping all data of interest in registers.  The compiler might
1581     therefore optimize the variable 'tmp' out of our previous example:
1582
1583        while (tmp = a)
1584                do_something_with(tmp);
1585
1586     This could result in the following code, which is perfectly safe in
1587     single-threaded code, but can be fatal in concurrent code:
1588
1589        while (a)
1590                do_something_with(a);
1591
1592     For example, the optimized version of this code could result in
1593     passing a zero to do_something_with() in the case where the variable
1594     a was modified by some other CPU between the "while" statement and
1595     the call to do_something_with().
1596
1597     Again, use READ_ONCE() to prevent the compiler from doing this:
1598
1599        while (tmp = READ_ONCE(a))
1600                do_something_with(tmp);
1601
1602     Note that if the compiler runs short of registers, it might save
1603     tmp onto the stack.  The overhead of this saving and later restoring
1604     is why compilers reload variables.  Doing so is perfectly safe for
1605     single-threaded code, so you need to tell the compiler about cases
1606     where it is not safe.
1607
1608 (*) The compiler is within its rights to omit a load entirely if it knows
1609     what the value will be.  For example, if the compiler can prove that
1610     the value of variable 'a' is always zero, it can optimize this code:
1611
1612        while (tmp = a)
1613                do_something_with(tmp);
1614
1615     Into this:
1616
1617        do { } while (0);
1618
1619     This transformation is a win for single-threaded code because it
1620     gets rid of a load and a branch.  The problem is that the compiler
1621     will carry out its proof assuming that the current CPU is the only
1622     one updating variable 'a'.  If variable 'a' is shared, then the
1623     compiler's proof will be erroneous.  Use READ_ONCE() to tell the
1624     compiler that it doesn't know as much as it thinks it does:
1625
1626        while (tmp = READ_ONCE(a))
1627                do_something_with(tmp);
1628
1629     But please note that the compiler is also closely watching what you
1630     do with the value after the READ_ONCE().  For example, suppose you
1631     do the following and MAX is a preprocessor macro with the value 1:
1632
1633        while ((tmp = READ_ONCE(a)) % MAX)
1634                do_something_with(tmp);
1635
1636     Then the compiler knows that the result of the "%" operator applied
1637     to MAX will always be zero, again allowing the compiler to optimize
1638     the code into near-nonexistence.  (It will still load from the
1639     variable 'a'.)
1640
1641 (*) Similarly, the compiler is within its rights to omit a store entirely
1642     if it knows that the variable already has the value being stored.
1643     Again, the compiler assumes that the current CPU is the only one
1644     storing into the variable, which can cause the compiler to do the
1645     wrong thing for shared variables.  For example, suppose you have
1646     the following:
1647
1648        a = 0;
1649        ... Code that does not store to variable a ...
1650        a = 0;
1651
1652     The compiler sees that the value of variable 'a' is already zero, so
1653     it might well omit the second store.  This would come as a fatal
1654     surprise if some other CPU might have stored to variable 'a' in the
1655     meantime.
1656
1657     Use WRITE_ONCE() to prevent the compiler from making this sort of
1658     wrong guess:
1659
1660        WRITE_ONCE(a, 0);
1661        ... Code that does not store to variable a ...
1662        WRITE_ONCE(a, 0);
1663
1664 (*) The compiler is within its rights to reorder memory accesses unless
1665     you tell it not to.  For example, consider the following interaction
1666     between process-level code and an interrupt handler:
1667
1668        void process_level(void)
1669        {
1670                msg = get_message();
1671                flag = true;
1672        }
1673
1674        void interrupt_handler(void)
1675        {
1676                if (flag)
1677                        process_message(msg);
1678        }
1679
1680     There is nothing to prevent the compiler from transforming
1681     process_level() to the following, in fact, this might well be a
1682     win for single-threaded code:
1683
1684        void process_level(void)
1685        {
1686                flag = true;
1687                msg = get_message();
1688        }
1689
1690     If the interrupt occurs between these two statement, then
1691     interrupt_handler() might be passed a garbled msg.  Use WRITE_ONCE()
1692     to prevent this as follows:
1693
1694        void process_level(void)
1695        {
1696                WRITE_ONCE(msg, get_message());
1697                WRITE_ONCE(flag, true);
1698        }
1699
1700        void interrupt_handler(void)
1701        {
1702                if (READ_ONCE(flag))
1703                        process_message(READ_ONCE(msg));
1704        }
1705
1706     Note that the READ_ONCE() and WRITE_ONCE() wrappers in
1707     interrupt_handler() are needed if this interrupt handler can itself
1708     be interrupted by something that also accesses 'flag' and 'msg',
1709     for example, a nested interrupt or an NMI.  Otherwise, READ_ONCE()
1710     and WRITE_ONCE() are not needed in interrupt_handler() other than
1711     for documentation purposes.  (Note also that nested interrupts
1712     do not typically occur in modern Linux kernels, in fact, if an
1713     interrupt handler returns with interrupts enabled, you will get a
1714     WARN_ONCE() splat.)
1715
1716     You should assume that the compiler can move READ_ONCE() and
1717     WRITE_ONCE() past code not containing READ_ONCE(), WRITE_ONCE(),
1718     barrier(), or similar primitives.
1719
1720     This effect could also be achieved using barrier(), but READ_ONCE()
1721     and WRITE_ONCE() are more selective:  With READ_ONCE() and
1722     WRITE_ONCE(), the compiler need only forget the contents of the
1723     indicated memory locations, while with barrier() the compiler must
1724     discard the value of all memory locations that it has currently
1725     cached in any machine registers.  Of course, the compiler must also
1726     respect the order in which the READ_ONCE()s and WRITE_ONCE()s occur,
1727     though the CPU of course need not do so.
1728
1729 (*) The compiler is within its rights to invent stores to a variable,
1730     as in the following example:
1731
1732        if (a)
1733                b = a;
1734        else
1735                b = 42;
1736
1737     The compiler might save a branch by optimizing this as follows:
1738
1739        b = 42;
1740        if (a)
1741                b = a;
1742
1743     In single-threaded code, this is not only safe, but also saves
1744     a branch.  Unfortunately, in concurrent code, this optimization
1745     could cause some other CPU to see a spurious value of 42 -- even
1746     if variable 'a' was never zero -- when loading variable 'b'.
1747     Use WRITE_ONCE() to prevent this as follows:
1748
1749        if (a)
1750                WRITE_ONCE(b, a);
1751        else
1752                WRITE_ONCE(b, 42);
1753
1754     The compiler can also invent loads.  These are usually less
1755     damaging, but they can result in cache-line bouncing and thus in
1756     poor performance and scalability.  Use READ_ONCE() to prevent
1757     invented loads.
1758
1759 (*) For aligned memory locations whose size allows them to be accessed
1760     with a single memory-reference instruction, prevents "load tearing"
1761     and "store tearing," in which a single large access is replaced by
1762     multiple smaller accesses.  For example, given an architecture having
1763     16-bit store instructions with 7-bit immediate fields, the compiler
1764     might be tempted to use two 16-bit store-immediate instructions to
1765     implement the following 32-bit store:
1766
1767        p = 0x00010002;
1768
1769     Please note that GCC really does use this sort of optimization,
1770     which is not surprising given that it would likely take more
1771     than two instructions to build the constant and then store it.
1772     This optimization can therefore be a win in single-threaded code.
1773     In fact, a recent bug (since fixed) caused GCC to incorrectly use
1774     this optimization in a volatile store.  In the absence of such bugs,
1775     use of WRITE_ONCE() prevents store tearing in the following example:
1776
1777        WRITE_ONCE(p, 0x00010002);
1778
1779     Use of packed structures can also result in load and store tearing,
1780     as in this example:
1781
1782        struct __attribute__((__packed__)) foo {
1783                short a;
1784                int b;
1785                short c;
1786        };
1787        struct foo foo1, foo2;
1788        ...
1789
1790        foo2.a = foo1.a;
1791        foo2.b = foo1.b;
1792        foo2.c = foo1.c;
1793
1794     Because there are no READ_ONCE() or WRITE_ONCE() wrappers and no
1795     volatile markings, the compiler would be well within its rights to
1796     implement these three assignment statements as a pair of 32-bit
1797     loads followed by a pair of 32-bit stores.  This would result in
1798     load tearing on 'foo1.b' and store tearing on 'foo2.b'.  READ_ONCE()
1799     and WRITE_ONCE() again prevent tearing in this example:
1800
1801        foo2.a = foo1.a;
1802        WRITE_ONCE(foo2.b, READ_ONCE(foo1.b));
1803        foo2.c = foo1.c;
1804
1805All that aside, it is never necessary to use READ_ONCE() and
1806WRITE_ONCE() on a variable that has been marked volatile.  For example,
1807because 'jiffies' is marked volatile, it is never necessary to
1808say READ_ONCE(jiffies).  The reason for this is that READ_ONCE() and
1809WRITE_ONCE() are implemented as volatile casts, which has no effect when
1810its argument is already marked volatile.
1811
1812Please note that these compiler barriers have no direct effect on the CPU,
1813which may then reorder things however it wishes.
1814
1815
1816CPU MEMORY BARRIERS
1817-------------------
1818
1819The Linux kernel has eight basic CPU memory barriers:
1820
1821        TYPE            MANDATORY               SMP CONDITIONAL
1822        =============== ======================= ===========================
1823        GENERAL         mb()                    smp_mb()
1824        WRITE           wmb()                   smp_wmb()
1825        READ            rmb()                   smp_rmb()
1826        DATA DEPENDENCY                         READ_ONCE()
1827
1828
1829All memory barriers except the data dependency barriers imply a compiler
1830barrier.  Data dependencies do not impose any additional compiler ordering.
1831
1832Aside: In the case of data dependencies, the compiler would be expected
1833to issue the loads in the correct order (eg. `a[b]` would have to load
1834the value of b before loading a[b]), however there is no guarantee in
1835the C specification that the compiler may not speculate the value of b
1836(eg. is equal to 1) and load a[b] before b (eg. tmp = a[1]; if (b != 1)
1837tmp = a[b]; ).  There is also the problem of a compiler reloading b after
1838having loaded a[b], thus having a newer copy of b than a[b].  A consensus
1839has not yet been reached about these problems, however the READ_ONCE()
1840macro is a good place to start looking.
1841
1842SMP memory barriers are reduced to compiler barriers on uniprocessor compiled
1843systems because it is assumed that a CPU will appear to be self-consistent,
1844and will order overlapping accesses correctly with respect to itself.
1845However, see the subsection on "Virtual Machine Guests" below.
1846
1847[!] Note that SMP memory barriers _must_ be used to control the ordering of
1848references to shared memory on SMP systems, though the use of locking instead
1849is sufficient.
1850
1851Mandatory barriers should not be used to control SMP effects, since mandatory
1852barriers impose unnecessary overhead on both SMP and UP systems. They may,
1853however, be used to control MMIO effects on accesses through relaxed memory I/O
1854windows.  These barriers are required even on non-SMP systems as they affect
1855the order in which memory operations appear to a device by prohibiting both the
1856compiler and the CPU from reordering them.
1857
1858
1859There are some more advanced barrier functions:
1860
1861 (*) smp_store_mb(var, value)
1862
1863     This assigns the value to the variable and then inserts a full memory
1864     barrier after it.  It isn't guaranteed to insert anything more than a
1865     compiler barrier in a UP compilation.
1866
1867
1868 (*) smp_mb__before_atomic();
1869 (*) smp_mb__after_atomic();
1870
1871     These are for use with atomic RMW functions that do not imply memory
1872     barriers, but where the code needs a memory barrier. Examples for atomic
1873     RMW functions that do not imply a memory barrier are e.g. add,
1874     subtract, (failed) conditional operations, _relaxed functions,
1875     but not atomic_read or atomic_set. A common example where a memory
1876     barrier may be required is when atomic ops are used for reference
1877     counting.
1878
1879     These are also used for atomic RMW bitop functions that do not imply a
1880     memory barrier (such as set_bit and clear_bit).
1881
1882     As an example, consider a piece of code that marks an object as being dead
1883     and then decrements the object's reference count:
1884
1885        obj->dead = 1;
1886        smp_mb__before_atomic();
1887        atomic_dec(&obj->ref_count);
1888
1889     This makes sure that the death mark on the object is perceived to be set
1890     *before* the reference counter is decremented.
1891
1892     See Documentation/atomic_{t,bitops}.txt for more information.
1893
1894
1895 (*) dma_wmb();
1896 (*) dma_rmb();
1897
1898     These are for use with consistent memory to guarantee the ordering
1899     of writes or reads of shared memory accessible to both the CPU and a
1900     DMA capable device.
1901
1902     For example, consider a device driver that shares memory with a device
1903     and uses a descriptor status value to indicate if the descriptor belongs
1904     to the device or the CPU, and a doorbell to notify it when new
1905     descriptors are available:
1906
1907        if (desc->status != DEVICE_OWN) {
1908                /* do not read data until we own descriptor */
1909                dma_rmb();
1910
1911                /* read/modify data */
1912                read_data = desc->data;
1913                desc->data = write_data;
1914
1915                /* flush modifications before status update */
1916                dma_wmb();
1917
1918                /* assign ownership */
1919                desc->status = DEVICE_OWN;
1920
1921                /* notify device of new descriptors */
1922                writel(DESC_NOTIFY, doorbell);
1923        }
1924
1925     The dma_rmb() allows us guarantee the device has released ownership
1926     before we read the data from the descriptor, and the dma_wmb() allows
1927     us to guarantee the data is written to the descriptor before the device
1928     can see it now has ownership.  Note that, when using writel(), a prior
1929     wmb() is not needed to guarantee that the cache coherent memory writes
1930     have completed before writing to the MMIO region.  The cheaper
1931     writel_relaxed() does not provide this guarantee and must not be used
1932     here.
1933
1934     See the subsection "Kernel I/O barrier effects" for more information on
1935     relaxed I/O accessors and the Documentation/core-api/dma-api.rst file for
1936     more information on consistent memory.
1937
1938 (*) pmem_wmb();
1939
1940     This is for use with persistent memory to ensure that stores for which
1941     modifications are written to persistent storage reached a platform
1942     durability domain.
1943
1944     For example, after a non-temporal write to pmem region, we use pmem_wmb()
1945     to ensure that stores have reached a platform durability domain. This ensures
1946     that stores have updated persistent storage before any data access or
1947     data transfer caused by subsequent instructions is initiated. This is
1948     in addition to the ordering done by wmb().
1949
1950     For load from persistent memory, existing read memory barriers are sufficient
1951     to ensure read ordering.
1952
1953===============================
1954IMPLICIT KERNEL MEMORY BARRIERS
1955===============================
1956
1957Some of the other functions in the linux kernel imply memory barriers, amongst
1958which are locking and scheduling functions.
1959
1960This specification is a _minimum_ guarantee; any particular architecture may
1961provide more substantial guarantees, but these may not be relied upon outside
1962of arch specific code.
1963
1964
1965LOCK ACQUISITION FUNCTIONS
1966--------------------------
1967
1968The Linux kernel has a number of locking constructs:
1969
1970 (*) spin locks
1971 (*) R/W spin locks
1972 (*) mutexes
1973 (*) semaphores
1974 (*) R/W semaphores
1975
1976In all cases there are variants on "ACQUIRE" operations and "RELEASE" operations
1977for each construct.  These operations all imply certain barriers:
1978
1979 (1) ACQUIRE operation implication:
1980
1981     Memory operations issued after the ACQUIRE will be completed after the
1982     ACQUIRE operation has completed.
1983
1984     Memory operations issued before the ACQUIRE may be completed after
1985     the ACQUIRE operation has completed.
1986
1987 (2) RELEASE operation implication:
1988
1989     Memory operations issued before the RELEASE will be completed before the
1990     RELEASE operation has completed.
1991
1992     Memory operations issued after the RELEASE may be completed before the
1993     RELEASE operation has completed.
1994
1995 (3) ACQUIRE vs ACQUIRE implication:
1996
1997     All ACQUIRE operations issued before another ACQUIRE operation will be
1998     completed before that ACQUIRE operation.
1999
2000 (4) ACQUIRE vs RELEASE implication:
2001
2002     All ACQUIRE operations issued before a RELEASE operation will be
2003     completed before the RELEASE operation.
2004
2005 (5) Failed conditional ACQUIRE implication:
2006
2007     Certain locking variants of the ACQUIRE operation may fail, either due to
2008     being unable to get the lock immediately, or due to receiving an unblocked
2009     signal while asleep waiting for the lock to become available.  Failed
2010     locks do not imply any sort of barrier.
2011
2012[!] Note: one of the consequences of lock ACQUIREs and RELEASEs being only
2013one-way barriers is that the effects of instructions outside of a critical
2014section may seep into the inside of the critical section.
2015
2016An ACQUIRE followed by a RELEASE may not be assumed to be full memory barrier
2017because it is possible for an access preceding the ACQUIRE to happen after the
2018ACQUIRE, and an access following the RELEASE to happen before the RELEASE, and
2019the two accesses can themselves then cross:
2020
2021        *A = a;
2022        ACQUIRE M
2023        RELEASE M
2024        *B = b;
2025
2026may occur as:
2027
2028        ACQUIRE M, STORE *B, STORE *A, RELEASE M
2029
2030When the ACQUIRE and RELEASE are a lock acquisition and release,
2031respectively, this same reordering can occur if the lock's ACQUIRE and
2032RELEASE are to the same lock variable, but only from the perspective of
2033another CPU not holding that lock.  In short, a ACQUIRE followed by an
2034RELEASE may -not- be assumed to be a full memory barrier.
2035
2036Similarly, the reverse case of a RELEASE followed by an ACQUIRE does
2037not imply a full memory barrier.  Therefore, the CPU's execution of the
2038critical sections corresponding to the RELEASE and the ACQUIRE can cross,
2039so that:
2040
2041        *A = a;
2042        RELEASE M
2043        ACQUIRE N
2044        *B = b;
2045
2046could occur as:
2047
2048        ACQUIRE N, STORE *B, STORE *A, RELEASE M
2049
2050It might appear that this reordering could introduce a deadlock.
2051However, this cannot happen because if such a deadlock threatened,
2052the RELEASE would simply complete, thereby avoiding the deadlock.
2053
2054        Why does this work?
2055
2056        One key point is that we are only talking about the CPU doing
2057        the reordering, not the compiler.  If the compiler (or, for
2058        that matter, the developer) switched the operations, deadlock
2059        -could- occur.
2060
2061        But suppose the CPU reordered the operations.  In this case,
2062        the unlock precedes the lock in the assembly code.  The CPU
2063        simply elected to try executing the later lock operation first.
2064        If there is a deadlock, this lock operation will simply spin (or
2065        try to sleep, but more on that later).  The CPU will eventually
2066        execute the unlock operation (which preceded the lock operation
2067        in the assembly code), which will unravel the potential deadlock,
2068        allowing the lock operation to succeed.
2069
2070        But what if the lock is a sleeplock?  In that case, the code will
2071        try to enter the scheduler, where it will eventually encounter
2072        a memory barrier, which will force the earlier unlock operation
2073        to complete, again unraveling the deadlock.  There might be
2074        a sleep-unlock race, but the locking primitive needs to resolve
2075        such races properly in any case.
2076
2077Locks and semaphores may not provide any guarantee of ordering on UP compiled
2078systems, and so cannot be counted on in such a situation to actually achieve
2079anything at all - especially with respect to I/O accesses - unless combined
2080with interrupt disabling operations.
2081
2082See also the section on "Inter-CPU acquiring barrier effects".
2083
2084
2085As an example, consider the following:
2086
2087        *A = a;
2088        *B = b;
2089        ACQUIRE
2090        *C = c;
2091        *D = d;
2092        RELEASE
2093        *E = e;
2094        *F = f;
2095
2096The following sequence of events is acceptable:
2097
2098        ACQUIRE, {*F,*A}, *E, {*C,*D}, *B, RELEASE
2099
2100        [+] Note that {*F,*A} indicates a combined access.
2101
2102But none of the following are:
2103
2104        {*F,*A}, *B,    ACQUIRE, *C, *D,        RELEASE, *E
2105        *A, *B, *C,     ACQUIRE, *D,            RELEASE, *E, *F
2106        *A, *B,         ACQUIRE, *C,            RELEASE, *D, *E, *F
2107        *B,             ACQUIRE, *C, *D,        RELEASE, {*F,*A}, *E
2108
2109
2110
2111INTERRUPT DISABLING FUNCTIONS
2112-----------------------------
2113
2114Functions that disable interrupts (ACQUIRE equivalent) and enable interrupts
2115(RELEASE equivalent) will act as compiler barriers only.  So if memory or I/O
2116barriers are required in such a situation, they must be provided from some
2117other means.
2118
2119
2120SLEEP AND WAKE-UP FUNCTIONS
2121---------------------------
2122
2123Sleeping and waking on an event flagged in global data can be viewed as an
2124interaction between two pieces of data: the task state of the task waiting for
2125the event and the global data used to indicate the event.  To make sure that
2126these appear to happen in the right order, the primitives to begin the process
2127of going to sleep, and the primitives to initiate a wake up imply certain
2128barriers.
2129
2130Firstly, the sleeper normally follows something like this sequence of events:
2131
2132        for (;;) {
2133                set_current_state(TASK_UNINTERRUPTIBLE);
2134                if (event_indicated)
2135                        break;
2136                schedule();
2137        }
2138
2139A general memory barrier is interpolated automatically by set_current_state()
2140after it has altered the task state:
2141
2142        CPU 1
2143        ===============================
2144        set_current_state();
2145          smp_store_mb();
2146            STORE current->state
2147            <general barrier>
2148        LOAD event_indicated
2149
2150set_current_state() may be wrapped by:
2151
2152        prepare_to_wait();
2153        prepare_to_wait_exclusive();
2154
2155which therefore also imply a general memory barrier after setting the state.
2156The whole sequence above is available in various canned forms, all of which
2157interpolate the memory barrier in the right place:
2158
2159        wait_event();
2160        wait_event_interruptible();
2161        wait_event_interruptible_exclusive();
2162        wait_event_interruptible_timeout();
2163        wait_event_killable();
2164        wait_event_timeout();
2165        wait_on_bit();
2166        wait_on_bit_lock();
2167
2168
2169Secondly, code that performs a wake up normally follows something like this:
2170
2171        event_indicated = 1;
2172        wake_up(&event_wait_queue);
2173
2174or:
2175
2176        event_indicated = 1;
2177        wake_up_process(event_daemon);
2178
2179A general memory barrier is executed by wake_up() if it wakes something up.
2180If it doesn't wake anything up then a memory barrier may or may not be
2181executed; you must not rely on it.  The barrier occurs before the task state
2182is accessed, in particular, it sits between the STORE to indicate the event
2183and the STORE to set TASK_RUNNING:
2184
2185        CPU 1 (Sleeper)                 CPU 2 (Waker)
2186        =============================== ===============================
2187        set_current_state();            STORE event_indicated
2188          smp_store_mb();               wake_up();
2189            STORE current->state          ...
2190            <general barrier>             <general barrier>
2191        LOAD event_indicated              if ((LOAD task->state) & TASK_NORMAL)
2192                                            STORE task->state
2193
2194where "task" is the thread being woken up and it equals CPU 1's "current".
2195
2196To repeat, a general memory barrier is guaranteed to be executed by wake_up()
2197if something is actually awakened, but otherwise there is no such guarantee.
2198To see this, consider the following sequence of events, where X and Y are both
2199initially zero:
2200
2201        CPU 1                           CPU 2
2202        =============================== ===============================
2203        X = 1;                          Y = 1;
2204        smp_mb();                       wake_up();
2205        LOAD Y                          LOAD X
2206
2207If a wakeup does occur, one (at least) of the two loads must see 1.  If, on
2208the other hand, a wakeup does not occur, both loads might see 0.
2209
2210wake_up_process() always executes a general memory barrier.  The barrier again
2211occurs before the task state is accessed.  In particular, if the wake_up() in
2212the previous snippet were replaced by a call to wake_up_process() then one of
2213the two loads would be guaranteed to see 1.
2214
2215The available waker functions include:
2216
2217        complete();
2218        wake_up();
2219        wake_up_all();
2220        wake_up_bit();
2221        wake_up_interruptible();
2222        wake_up_interruptible_all();
2223        wake_up_interruptible_nr();
2224        wake_up_interruptible_poll();
2225        wake_up_interruptible_sync();
2226        wake_up_interruptible_sync_poll();
2227        wake_up_locked();
2228        wake_up_locked_poll();
2229        wake_up_nr();
2230        wake_up_poll();
2231        wake_up_process();
2232
2233In terms of memory ordering, these functions all provide the same guarantees of
2234a wake_up() (or stronger).
2235
2236[!] Note that the memory barriers implied by the sleeper and the waker do _not_
2237order multiple stores before the wake-up with respect to loads of those stored
2238values after the sleeper has called set_current_state().  For instance, if the
2239sleeper does:
2240
2241        set_current_state(TASK_INTERRUPTIBLE);
2242        if (event_indicated)
2243                break;
2244        __set_current_state(TASK_RUNNING);
2245        do_something(my_data);
2246
2247and the waker does:
2248
2249        my_data = value;
2250        event_indicated = 1;
2251        wake_up(&event_wait_queue);
2252
2253there's no guarantee that the change to event_indicated will be perceived by
2254the sleeper as coming after the change to my_data.  In such a circumstance, the
2255code on both sides must interpolate its own memory barriers between the
2256separate data accesses.  Thus the above sleeper ought to do:
2257
2258        set_current_state(TASK_INTERRUPTIBLE);
2259        if (event_indicated) {
2260                smp_rmb();
2261                do_something(my_data);
2262        }
2263
2264and the waker should do:
2265
2266        my_data = value;
2267        smp_wmb();
2268        event_indicated = 1;
2269        wake_up(&event_wait_queue);
2270
2271
2272MISCELLANEOUS FUNCTIONS
2273-----------------------
2274
2275Other functions that imply barriers:
2276
2277 (*) schedule() and similar imply full memory barriers.
2278
2279
2280===================================
2281INTER-CPU ACQUIRING BARRIER EFFECTS
2282===================================
2283
2284On SMP systems locking primitives give a more substantial form of barrier: one
2285that does affect memory access ordering on other CPUs, within the context of
2286conflict on any particular lock.
2287
2288
2289ACQUIRES VS MEMORY ACCESSES
2290---------------------------
2291
2292Consider the following: the system has a pair of spinlocks (M) and (Q), and
2293three CPUs; then should the following sequence of events occur:
2294
2295        CPU 1                           CPU 2
2296        =============================== ===============================
2297        WRITE_ONCE(*A, a);              WRITE_ONCE(*E, e);
2298        ACQUIRE M                       ACQUIRE Q
2299        WRITE_ONCE(*B, b);              WRITE_ONCE(*F, f);
2300        WRITE_ONCE(*C, c);              WRITE_ONCE(*G, g);
2301        RELEASE M                       RELEASE Q
2302        WRITE_ONCE(*D, d);              WRITE_ONCE(*H, h);
2303
2304Then there is no guarantee as to what order CPU 3 will see the accesses to *A
2305through *H occur in, other than the constraints imposed by the separate locks
2306on the separate CPUs.  It might, for example, see:
2307
2308        *E, ACQUIRE M, ACQUIRE Q, *G, *C, *F, *A, *B, RELEASE Q, *D, *H, RELEASE M
2309
2310But it won't see any of:
2311
2312        *B, *C or *D preceding ACQUIRE M
2313        *A, *B or *C following RELEASE M
2314        *F, *G or *H preceding ACQUIRE Q
2315        *E, *F or *G following RELEASE Q
2316
2317
2318=================================
2319WHERE ARE MEMORY BARRIERS NEEDED?
2320=================================
2321
2322Under normal operation, memory operation reordering is generally not going to
2323be a problem as a single-threaded linear piece of code will still appear to
2324work correctly, even if it's in an SMP kernel.  There are, however, four
2325circumstances in which reordering definitely _could_ be a problem:
2326
2327 (*) Interprocessor interaction.
2328
2329 (*) Atomic operations.
2330
2331 (*) Accessing devices.
2332
2333 (*) Interrupts.
2334
2335
2336INTERPROCESSOR INTERACTION
2337--------------------------
2338
2339When there's a system with more than one processor, more than one CPU in the
2340system may be working on the same data set at the same time.  This can cause
2341synchronisation problems, and the usual way of dealing with them is to use
2342locks.  Locks, however, are quite expensive, and so it may be preferable to
2343operate without the use of a lock if at all possible.  In such a case
2344operations that affect both CPUs may have to be carefully ordered to prevent
2345a malfunction.
2346
2347Consider, for example, the R/W semaphore slow path.  Here a waiting process is
2348queued on the semaphore, by virtue of it having a piece of its stack linked to
2349the semaphore's list of waiting processes:
2350
2351        struct rw_semaphore {
2352                ...
2353                spinlock_t lock;
2354                struct list_head waiters;
2355        };
2356
2357        struct rwsem_waiter {
2358                struct list_head list;
2359                struct task_struct *task;
2360        };
2361
2362To wake up a particular waiter, the up_read() or up_write() functions have to:
2363
2364 (1) read the next pointer from this waiter's record to know as to where the
2365     next waiter record is;
2366
2367 (2) read the pointer to the waiter's task structure;
2368
2369 (3) clear the task pointer to tell the waiter it has been given the semaphore;
2370
2371 (4) call wake_up_process() on the task; and
2372
2373 (5) release the reference held on the waiter's task struct.
2374
2375In other words, it has to perform this sequence of events:
2376
2377        LOAD waiter->list.next;
2378        LOAD waiter->task;
2379        STORE waiter->task;
2380        CALL wakeup
2381        RELEASE task
2382
2383and if any of these steps occur out of order, then the whole thing may
2384malfunction.
2385
2386Once it has queued itself and dropped the semaphore lock, the waiter does not
2387get the lock again; it instead just waits for its task pointer to be cleared
2388before proceeding.  Since the record is on the waiter's stack, this means that
2389if the task pointer is cleared _before_ the next pointer in the list is read,
2390another CPU might start processing the waiter and might clobber the waiter's
2391stack before the up*() function has a chance to read the next pointer.
2392
2393Consider then what might happen to the above sequence of events:
2394
2395        CPU 1                           CPU 2
2396        =============================== ===============================
2397                                        down_xxx()
2398                                        Queue waiter
2399                                        Sleep
2400        up_yyy()
2401        LOAD waiter->task;
2402        STORE waiter->task;
2403                                        Woken up by other event
2404        <preempt>
2405                                        Resume processing
2406                                        down_xxx() returns
2407                                        call foo()
2408                                        foo() clobbers *waiter
2409        </preempt>
2410        LOAD waiter->list.next;
2411        --- OOPS ---
2412
2413This could be dealt with using the semaphore lock, but then the down_xxx()
2414function has to needlessly get the spinlock again after being woken up.
2415
2416The way to deal with this is to insert a general SMP memory barrier:
2417
2418        LOAD waiter->list.next;
2419        LOAD waiter->task;
2420        smp_mb();
2421        STORE waiter->task;
2422        CALL wakeup
2423        RELEASE task
2424
2425In this case, the barrier makes a guarantee that all memory accesses before the
2426barrier will appear to happen before all the memory accesses after the barrier
2427with respect to the other CPUs on the system.  It does _not_ guarantee that all
2428the memory accesses before the barrier will be complete by the time the barrier
2429instruction itself is complete.
2430
2431On a UP system - where this wouldn't be a problem - the smp_mb() is just a
2432compiler barrier, thus making sure the compiler emits the instructions in the
2433right order without actually intervening in the CPU.  Since there's only one
2434CPU, that CPU's dependency ordering logic will take care of everything else.
2435
2436
2437ATOMIC OPERATIONS
2438-----------------
2439
2440While they are technically interprocessor interaction considerations, atomic
2441operations are noted specially as some of them imply full memory barriers and
2442some don't, but they're very heavily relied on as a group throughout the
2443kernel.
2444
2445See Documentation/atomic_t.txt for more information.
2446
2447
2448ACCESSING DEVICES
2449-----------------
2450
2451Many devices can be memory mapped, and so appear to the CPU as if they're just
2452a set of memory locations.  To control such a device, the driver usually has to
2453make the right memory accesses in exactly the right order.
2454
2455However, having a clever CPU or a clever compiler creates a potential problem
2456in that the carefully sequenced accesses in the driver code won't reach the
2457device in the requisite order if the CPU or the compiler thinks it is more
2458efficient to reorder, combine or merge accesses - something that would cause
2459the device to malfunction.
2460
2461Inside of the Linux kernel, I/O should be done through the appropriate accessor
2462routines - such as inb() or writel() - which know how to make such accesses
2463appropriately sequential.  While this, for the most part, renders the explicit
2464use of memory barriers unnecessary, if the accessor functions are used to refer
2465to an I/O memory window with relaxed memory access properties, then _mandatory_
2466memory barriers are required to enforce ordering.
2467
2468See Documentation/driver-api/device-io.rst for more information.
2469
2470
2471INTERRUPTS
2472----------
2473
2474A driver may be interrupted by its own interrupt service routine, and thus the
2475two parts of the driver may interfere with each other's attempts to control or
2476access the device.
2477
2478This may be alleviated - at least in part - by disabling local interrupts (a
2479form of locking), such that the critical operations are all contained within
2480the interrupt-disabled section in the driver.  While the driver's interrupt
2481routine is executing, the driver's core may not run on the same CPU, and its
2482interrupt is not permitted to happen again until the current interrupt has been
2483handled, thus the interrupt handler does not need to lock against that.
2484
2485However, consider a driver that was talking to an ethernet card that sports an
2486address register and a data register.  If that driver's core talks to the card
2487under interrupt-disablement and then the driver's interrupt handler is invoked:
2488
2489        LOCAL IRQ DISABLE
2490        writew(ADDR, 3);
2491        writew(DATA, y);
2492        LOCAL IRQ ENABLE
2493        <interrupt>
2494        writew(ADDR, 4);
2495        q = readw(DATA);
2496        </interrupt>
2497
2498The store to the data register might happen after the second store to the
2499address register if ordering rules are sufficiently relaxed:
2500
2501        STORE *ADDR = 3, STORE *ADDR = 4, STORE *DATA = y, q = LOAD *DATA
2502
2503
2504If ordering rules are relaxed, it must be assumed that accesses done inside an
2505interrupt disabled section may leak outside of it and may interleave with
2506accesses performed in an interrupt - and vice versa - unless implicit or
2507explicit barriers are used.
2508
2509Normally this won't be a problem because the I/O accesses done inside such
2510sections will include synchronous load operations on strictly ordered I/O
2511registers that form implicit I/O barriers.
2512
2513
2514A similar situation may occur between an interrupt routine and two routines
2515running on separate CPUs that communicate with each other.  If such a case is
2516likely, then interrupt-disabling locks should be used to guarantee ordering.
2517
2518
2519==========================
2520KERNEL I/O BARRIER EFFECTS
2521==========================
2522
2523Interfacing with peripherals via I/O accesses is deeply architecture and device
2524specific. Therefore, drivers which are inherently non-portable may rely on
2525specific behaviours of their target systems in order to achieve synchronization
2526in the most lightweight manner possible. For drivers intending to be portable
2527between multiple architectures and bus implementations, the kernel offers a
2528series of accessor functions that provide various degrees of ordering
2529guarantees:
2530
2531 (*) readX(), writeX():
2532
2533        The readX() and writeX() MMIO accessors take a pointer to the
2534        peripheral being accessed as an __iomem * parameter. For pointers
2535        mapped with the default I/O attributes (e.g. those returned by
2536        ioremap()), the ordering guarantees are as follows:
2537
2538        1. All readX() and writeX() accesses to the same peripheral are ordered
2539           with respect to each other. This ensures that MMIO register accesses
2540           by the same CPU thread to a particular device will arrive in program
2541           order.
2542
2543        2. A writeX() issued by a CPU thread holding a spinlock is ordered
2544           before a writeX() to the same peripheral from another CPU thread
2545           issued after a later acquisition of the same spinlock. This ensures
2546           that MMIO register writes to a particular device issued while holding
2547           a spinlock will arrive in an order consistent with acquisitions of
2548           the lock.
2549
2550        3. A writeX() by a CPU thread to the peripheral will first wait for the
2551           completion of all prior writes to memory either issued by, or
2552           propagated to, the same thread. This ensures that writes by the CPU
2553           to an outbound DMA buffer allocated by dma_alloc_coherent() will be
2554           visible to a DMA engine when the CPU writes to its MMIO control
2555           register to trigger the transfer.
2556
2557        4. A readX() by a CPU thread from the peripheral will complete before
2558           any subsequent reads from memory by the same thread can begin. This
2559           ensures that reads by the CPU from an incoming DMA buffer allocated
2560           by dma_alloc_coherent() will not see stale data after reading from
2561           the DMA engine's MMIO status register to establish that the DMA
2562           transfer has completed.
2563
2564        5. A readX() by a CPU thread from the peripheral will complete before
2565           any subsequent delay() loop can begin execution on the same thread.
2566           This ensures that two MMIO register writes by the CPU to a peripheral
2567           will arrive at least 1us apart if the first write is immediately read
2568           back with readX() and udelay(1) is called prior to the second
2569           writeX():
2570
2571                writel(42, DEVICE_REGISTER_0); // Arrives at the device...
2572                readl(DEVICE_REGISTER_0);
2573                udelay(1);
2574                writel(42, DEVICE_REGISTER_1); // ...at least 1us before this.
2575
2576        The ordering properties of __iomem pointers obtained with non-default
2577        attributes (e.g. those returned by ioremap_wc()) are specific to the
2578        underlying architecture and therefore the guarantees listed above cannot
2579        generally be relied upon for accesses to these types of mappings.
2580
2581 (*) readX_relaxed(), writeX_relaxed():
2582
2583        These are similar to readX() and writeX(), but provide weaker memory
2584        ordering guarantees. Specifically, they do not guarantee ordering with
2585        respect to locking, normal memory accesses or delay() loops (i.e.
2586        bullets 2-5 above) but they are still guaranteed to be ordered with
2587        respect to other accesses from the same CPU thread to the same
2588        peripheral when operating on __iomem pointers mapped with the default
2589        I/O attributes.
2590
2591 (*) readsX(), writesX():
2592
2593        The readsX() and writesX() MMIO accessors are designed for accessing
2594        register-based, memory-mapped FIFOs residing on peripherals that are not
2595        capable of performing DMA. Consequently, they provide only the ordering
2596        guarantees of readX_relaxed() and writeX_relaxed(), as documented above.
2597
2598 (*) inX(), outX():
2599
2600        The inX() and outX() accessors are intended to access legacy port-mapped
2601        I/O peripherals, which may require special instructions on some
2602        architectures (notably x86). The port number of the peripheral being
2603        accessed is passed as an argument.
2604
2605        Since many CPU architectures ultimately access these peripherals via an
2606        internal virtual memory mapping, the portable ordering guarantees
2607        provided by inX() and outX() are the same as those provided by readX()
2608        and writeX() respectively when accessing a mapping with the default I/O
2609        attributes.
2610
2611        Device drivers may expect outX() to emit a non-posted write transaction
2612        that waits for a completion response from the I/O peripheral before
2613        returning. This is not guaranteed by all architectures and is therefore
2614        not part of the portable ordering semantics.
2615
2616 (*) insX(), outsX():
2617
2618        As above, the insX() and outsX() accessors provide the same ordering
2619        guarantees as readsX() and writesX() respectively when accessing a
2620        mapping with the default I/O attributes.
2621
2622 (*) ioreadX(), iowriteX():
2623
2624        These will perform appropriately for the type of access they're actually
2625        doing, be it inX()/outX() or readX()/writeX().
2626
2627With the exception of the string accessors (insX(), outsX(), readsX() and
2628writesX()), all of the above assume that the underlying peripheral is
2629little-endian and will therefore perform byte-swapping operations on big-endian
2630architectures.
2631
2632
2633========================================
2634ASSUMED MINIMUM EXECUTION ORDERING MODEL
2635========================================
2636
2637It has to be assumed that the conceptual CPU is weakly-ordered but that it will
2638maintain the appearance of program causality with respect to itself.  Some CPUs
2639(such as i386 or x86_64) are more constrained than others (such as powerpc or
2640frv), and so the most relaxed case (namely DEC Alpha) must be assumed outside
2641of arch-specific code.
2642
2643This means that it must be considered that the CPU will execute its instruction
2644stream in any order it feels like - or even in parallel - provided that if an
2645instruction in the stream depends on an earlier instruction, then that
2646earlier instruction must be sufficiently complete[*] before the later
2647instruction may proceed; in other words: provided that the appearance of
2648causality is maintained.
2649
2650 [*] Some instructions have more than one effect - such as changing the
2651     condition codes, changing registers or changing memory - and different
2652     instructions may depend on different effects.
2653
2654A CPU may also discard any instruction sequence that winds up having no
2655ultimate effect.  For example, if two adjacent instructions both load an
2656immediate value into the same register, the first may be discarded.
2657
2658
2659Similarly, it has to be assumed that compiler might reorder the instruction
2660stream in any way it sees fit, again provided the appearance of causality is
2661maintained.
2662
2663
2664============================
2665THE EFFECTS OF THE CPU CACHE
2666============================
2667
2668The way cached memory operations are perceived across the system is affected to
2669a certain extent by the caches that lie between CPUs and memory, and by the
2670memory coherence system that maintains the consistency of state in the system.
2671
2672As far as the way a CPU interacts with another part of the system through the
2673caches goes, the memory system has to include the CPU's caches, and memory
2674barriers for the most part act at the interface between the CPU and its cache
2675(memory barriers logically act on the dotted line in the following diagram):
2676
2677            <--- CPU --->         :       <----------- Memory ----------->
2678                                  :
2679        +--------+    +--------+  :   +--------+    +-----------+
2680        |        |    |        |  :   |        |    |           |    +--------+
2681        |  CPU   |    | Memory |  :   | CPU    |    |           |    |        |
2682        |  Core  |--->| Access |----->| Cache  |<-->|           |    |        |
2683        |        |    | Queue  |  :   |        |    |           |--->| Memory |
2684        |        |    |        |  :   |        |    |           |    |        |
2685        +--------+    +--------+  :   +--------+    |           |    |        |
2686                                  :                 | Cache     |    +--------+
2687                                  :                 | Coherency |
2688                                  :                 | Mechanism |    +--------+
2689        +--------+    +--------+  :   +--------+    |           |    |        |
2690        |        |    |        |  :   |        |    |           |    |        |
2691        |  CPU   |    | Memory |  :   | CPU    |    |           |--->| Device |
2692        |  Core  |--->| Access |----->| Cache  |<-->|           |    |        |
2693        |        |    | Queue  |  :   |        |    |           |    |        |
2694        |        |    |        |  :   |        |    |           |    +--------+
2695        +--------+    +--------+  :   +--------+    +-----------+
2696                                  :
2697                                  :
2698
2699Although any particular load or store may not actually appear outside of the
2700CPU that issued it since it may have been satisfied within the CPU's own cache,
2701it will still appear as if the full memory access had taken place as far as the
2702other CPUs are concerned since the cache coherency mechanisms will migrate the
2703cacheline over to the accessing CPU and propagate the effects upon conflict.
2704
2705The CPU core may execute instructions in any order it deems fit, provided the
2706expected program causality appears to be maintained.  Some of the instructions
2707generate load and store operations which then go into the queue of memory
2708accesses to be performed.  The core may place these in the queue in any order
2709it wishes, and continue execution until it is forced to wait for an instruction
2710to complete.
2711
2712What memory barriers are concerned with is controlling the order in which
2713accesses cross from the CPU side of things to the memory side of things, and
2714the order in which the effects are perceived to happen by the other observers
2715in the system.
2716
2717[!] Memory barriers are _not_ needed within a given CPU, as CPUs always see
2718their own loads and stores as if they had happened in program order.
2719
2720[!] MMIO or other device accesses may bypass the cache system.  This depends on
2721the properties of the memory window through which devices are accessed and/or
2722the use of any special device communication instructions the CPU may have.
2723
2724
2725CACHE COHERENCY VS DMA
2726----------------------
2727
2728Not all systems maintain cache coherency with respect to devices doing DMA.  In
2729such cases, a device attempting DMA may obtain stale data from RAM because
2730dirty cache lines may be resident in the caches of various CPUs, and may not
2731have been written back to RAM yet.  To deal with this, the appropriate part of
2732the kernel must flush the overlapping bits of cache on each CPU (and maybe
2733invalidate them as well).
2734
2735In addition, the data DMA'd to RAM by a device may be overwritten by dirty
2736cache lines being written back to RAM from a CPU's cache after the device has
2737installed its own data, or cache lines present in the CPU's cache may simply
2738obscure the fact that RAM has been updated, until at such time as the cacheline
2739is discarded from the CPU's cache and reloaded.  To deal with this, the
2740appropriate part of the kernel must invalidate the overlapping bits of the
2741cache on each CPU.
2742
2743See Documentation/core-api/cachetlb.rst for more information on cache management.
2744
2745
2746CACHE COHERENCY VS MMIO
2747-----------------------
2748
2749Memory mapped I/O usually takes place through memory locations that are part of
2750a window in the CPU's memory space that has different properties assigned than
2751the usual RAM directed window.
2752
2753Amongst these properties is usually the fact that such accesses bypass the
2754caching entirely and go directly to the device buses.  This means MMIO accesses
2755may, in effect, overtake accesses to cached memory that were emitted earlier.
2756A memory barrier isn't sufficient in such a case, but rather the cache must be
2757flushed between the cached memory write and the MMIO access if the two are in
2758any way dependent.
2759
2760
2761=========================
2762THE THINGS CPUS GET UP TO
2763=========================
2764
2765A programmer might take it for granted that the CPU will perform memory
2766operations in exactly the order specified, so that if the CPU is, for example,
2767given the following piece of code to execute:
2768
2769        a = READ_ONCE(*A);
2770        WRITE_ONCE(*B, b);
2771        c = READ_ONCE(*C);
2772        d = READ_ONCE(*D);
2773        WRITE_ONCE(*E, e);
2774
2775they would then expect that the CPU will complete the memory operation for each
2776instruction before moving on to the next one, leading to a definite sequence of
2777operations as seen by external observers in the system:
2778
2779        LOAD *A, STORE *B, LOAD *C, LOAD *D, STORE *E.
2780
2781
2782Reality is, of course, much messier.  With many CPUs and compilers, the above
2783assumption doesn't hold because:
2784
2785 (*) loads are more likely to need to be completed immediately to permit
2786     execution progress, whereas stores can often be deferred without a
2787     problem;
2788
2789 (*) loads may be done speculatively, and the result discarded should it prove
2790     to have been unnecessary;
2791
2792 (*) loads may be done speculatively, leading to the result having been fetched
2793     at the wrong time in the expected sequence of events;
2794
2795 (*) the order of the memory accesses may be rearranged to promote better use
2796     of the CPU buses and caches;
2797
2798 (*) loads and stores may be combined to improve performance when talking to
2799     memory or I/O hardware that can do batched accesses of adjacent locations,
2800     thus cutting down on transaction setup costs (memory and PCI devices may
2801     both be able to do this); and
2802
2803 (*) the CPU's data cache may affect the ordering, and while cache-coherency
2804     mechanisms may alleviate this - once the store has actually hit the cache
2805     - there's no guarantee that the coherency management will be propagated in
2806     order to other CPUs.
2807
2808So what another CPU, say, might actually observe from the above piece of code
2809is:
2810
2811        LOAD *A, ..., LOAD {*C,*D}, STORE *E, STORE *B
2812
2813        (Where "LOAD {*C,*D}" is a combined load)
2814
2815
2816However, it is guaranteed that a CPU will be self-consistent: it will see its
2817_own_ accesses appear to be correctly ordered, without the need for a memory
2818barrier.  For instance with the following code:
2819
2820        U = READ_ONCE(*A);
2821        WRITE_ONCE(*A, V);
2822        WRITE_ONCE(*A, W);
2823        X = READ_ONCE(*A);
2824        WRITE_ONCE(*A, Y);
2825        Z = READ_ONCE(*A);
2826
2827and assuming no intervention by an external influence, it can be assumed that
2828the final result will appear to be:
2829
2830        U == the original value of *A
2831        X == W
2832        Z == Y
2833        *A == Y
2834
2835The code above may cause the CPU to generate the full sequence of memory
2836accesses:
2837
2838        U=LOAD *A, STORE *A=V, STORE *A=W, X=LOAD *A, STORE *A=Y, Z=LOAD *A
2839
2840in that order, but, without intervention, the sequence may have almost any
2841combination of elements combined or discarded, provided the program's view
2842of the world remains consistent.  Note that READ_ONCE() and WRITE_ONCE()
2843are -not- optional in the above example, as there are architectures
2844where a given CPU might reorder successive loads to the same location.
2845On such architectures, READ_ONCE() and WRITE_ONCE() do whatever is
2846necessary to prevent this, for example, on Itanium the volatile casts
2847used by READ_ONCE() and WRITE_ONCE() cause GCC to emit the special ld.acq
2848and st.rel instructions (respectively) that prevent such reordering.
2849
2850The compiler may also combine, discard or defer elements of the sequence before
2851the CPU even sees them.
2852
2853For instance:
2854
2855        *A = V;
2856        *A = W;
2857
2858may be reduced to:
2859
2860        *A = W;
2861
2862since, without either a write barrier or an WRITE_ONCE(), it can be
2863assumed that the effect of the storage of V to *A is lost.  Similarly:
2864
2865        *A = Y;
2866        Z = *A;
2867
2868may, without a memory barrier or an READ_ONCE() and WRITE_ONCE(), be
2869reduced to:
2870
2871        *A = Y;
2872        Z = Y;
2873
2874and the LOAD operation never appear outside of the CPU.
2875
2876
2877AND THEN THERE'S THE ALPHA
2878--------------------------
2879
2880The DEC Alpha CPU is one of the most relaxed CPUs there is.  Not only that,
2881some versions of the Alpha CPU have a split data cache, permitting them to have
2882two semantically-related cache lines updated at separate times.  This is where
2883the data dependency barrier really becomes necessary as this synchronises both
2884caches with the memory coherence system, thus making it seem like pointer
2885changes vs new data occur in the right order.
2886
2887The Alpha defines the Linux kernel's memory model, although as of v4.15
2888the Linux kernel's addition of smp_mb() to READ_ONCE() on Alpha greatly
2889reduced its impact on the memory model.
2890
2891
2892VIRTUAL MACHINE GUESTS
2893----------------------
2894
2895Guests running within virtual machines might be affected by SMP effects even if
2896the guest itself is compiled without SMP support.  This is an artifact of
2897interfacing with an SMP host while running an UP kernel.  Using mandatory
2898barriers for this use-case would be possible but is often suboptimal.
2899
2900To handle this case optimally, low-level virt_mb() etc macros are available.
2901These have the same effect as smp_mb() etc when SMP is enabled, but generate
2902identical code for SMP and non-SMP systems.  For example, virtual machine guests
2903should use virt_mb() rather than smp_mb() when synchronizing against a
2904(possibly SMP) host.
2905
2906These are equivalent to smp_mb() etc counterparts in all other respects,
2907in particular, they do not control MMIO effects: to control
2908MMIO effects, use mandatory barriers.
2909
2910
2911============
2912EXAMPLE USES
2913============
2914
2915CIRCULAR BUFFERS
2916----------------
2917
2918Memory barriers can be used to implement circular buffering without the need
2919of a lock to serialise the producer with the consumer.  See:
2920
2921        Documentation/core-api/circular-buffers.rst
2922
2923for details.
2924
2925
2926==========
2927REFERENCES
2928==========
2929
2930Alpha AXP Architecture Reference Manual, Second Edition (Sites & Witek,
2931Digital Press)
2932        Chapter 5.2: Physical Address Space Characteristics
2933        Chapter 5.4: Caches and Write Buffers
2934        Chapter 5.5: Data Sharing
2935        Chapter 5.6: Read/Write Ordering
2936
2937AMD64 Architecture Programmer's Manual Volume 2: System Programming
2938        Chapter 7.1: Memory-Access Ordering
2939        Chapter 7.4: Buffering and Combining Memory Writes
2940
2941ARM Architecture Reference Manual (ARMv8, for ARMv8-A architecture profile)
2942        Chapter B2: The AArch64 Application Level Memory Model
2943
2944IA-32 Intel Architecture Software Developer's Manual, Volume 3:
2945System Programming Guide
2946        Chapter 7.1: Locked Atomic Operations
2947        Chapter 7.2: Memory Ordering
2948        Chapter 7.4: Serializing Instructions
2949
2950The SPARC Architecture Manual, Version 9
2951        Chapter 8: Memory Models
2952        Appendix D: Formal Specification of the Memory Models
2953        Appendix J: Programming with the Memory Models
2954
2955Storage in the PowerPC (Stone and Fitzgerald)
2956
2957UltraSPARC Programmer Reference Manual
2958        Chapter 5: Memory Accesses and Cacheability
2959        Chapter 15: Sparc-V9 Memory Models
2960
2961UltraSPARC III Cu User's Manual
2962        Chapter 9: Memory Models
2963
2964UltraSPARC IIIi Processor User's Manual
2965        Chapter 8: Memory Models
2966
2967UltraSPARC Architecture 2005
2968        Chapter 9: Memory
2969        Appendix D: Formal Specifications of the Memory Models
2970
2971UltraSPARC T1 Supplement to the UltraSPARC Architecture 2005
2972        Chapter 8: Memory Models
2973        Appendix F: Caches and Cache Coherency
2974
2975Solaris Internals, Core Kernel Architecture, p63-68:
2976        Chapter 3.3: Hardware Considerations for Locks and
2977                        Synchronization
2978
2979Unix Systems for Modern Architectures, Symmetric Multiprocessing and Caching
2980for Kernel Programmers:
2981        Chapter 13: Other Memory Models
2982
2983Intel Itanium Architecture Software Developer's Manual: Volume 1:
2984        Section 2.6: Speculation
2985        Section 4.4: Memory Access
2986