linux/Documentation/preempt-locking.txt
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   1===========================================================================
   2Proper Locking Under a Preemptible Kernel: Keeping Kernel Code Preempt-Safe
   3===========================================================================
   4
   5:Author: Robert Love <rml@tech9.net>
   6:Last Updated: 28 Aug 2002
   7
   8
   9Introduction
  10============
  11
  12
  13A preemptible kernel creates new locking issues.  The issues are the same as
  14those under SMP: concurrency and reentrancy.  Thankfully, the Linux preemptible
  15kernel model leverages existing SMP locking mechanisms.  Thus, the kernel
  16requires explicit additional locking for very few additional situations.
  17
  18This document is for all kernel hackers.  Developing code in the kernel
  19requires protecting these situations.
  20 
  21
  22RULE #1: Per-CPU data structures need explicit protection
  23^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  24
  25
  26Two similar problems arise. An example code snippet::
  27
  28        struct this_needs_locking tux[NR_CPUS];
  29        tux[smp_processor_id()] = some_value;
  30        /* task is preempted here... */
  31        something = tux[smp_processor_id()];
  32
  33First, since the data is per-CPU, it may not have explicit SMP locking, but
  34require it otherwise.  Second, when a preempted task is finally rescheduled,
  35the previous value of smp_processor_id may not equal the current.  You must
  36protect these situations by disabling preemption around them.
  37
  38You can also use put_cpu() and get_cpu(), which will disable preemption.
  39
  40
  41RULE #2: CPU state must be protected.
  42^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  43
  44
  45Under preemption, the state of the CPU must be protected.  This is arch-
  46dependent, but includes CPU structures and state not preserved over a context
  47switch.  For example, on x86, entering and exiting FPU mode is now a critical
  48section that must occur while preemption is disabled.  Think what would happen
  49if the kernel is executing a floating-point instruction and is then preempted.
  50Remember, the kernel does not save FPU state except for user tasks.  Therefore,
  51upon preemption, the FPU registers will be sold to the lowest bidder.  Thus,
  52preemption must be disabled around such regions.
  53
  54Note, some FPU functions are already explicitly preempt safe.  For example,
  55kernel_fpu_begin and kernel_fpu_end will disable and enable preemption.
  56However, fpu__restore() must be called with preemption disabled.
  57
  58
  59RULE #3: Lock acquire and release must be performed by same task
  60^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  61
  62
  63A lock acquired in one task must be released by the same task.  This
  64means you can't do oddball things like acquire a lock and go off to
  65play while another task releases it.  If you want to do something
  66like this, acquire and release the task in the same code path and
  67have the caller wait on an event by the other task.
  68
  69
  70Solution
  71========
  72
  73
  74Data protection under preemption is achieved by disabling preemption for the
  75duration of the critical region.
  76
  77::
  78
  79  preempt_enable()              decrement the preempt counter
  80  preempt_disable()             increment the preempt counter
  81  preempt_enable_no_resched()   decrement, but do not immediately preempt
  82  preempt_check_resched()       if needed, reschedule
  83  preempt_count()               return the preempt counter
  84
  85The functions are nestable.  In other words, you can call preempt_disable
  86n-times in a code path, and preemption will not be reenabled until the n-th
  87call to preempt_enable.  The preempt statements define to nothing if
  88preemption is not enabled.
  89
  90Note that you do not need to explicitly prevent preemption if you are holding
  91any locks or interrupts are disabled, since preemption is implicitly disabled
  92in those cases.
  93
  94But keep in mind that 'irqs disabled' is a fundamentally unsafe way of
  95disabling preemption - any spin_unlock() decreasing the preemption count
  96to 0 might trigger a reschedule. A simple printk() might trigger a reschedule.
  97So use this implicit preemption-disabling property only if you know that the
  98affected codepath does not do any of this. Best policy is to use this only for
  99small, atomic code that you wrote and which calls no complex functions.
 100
 101Example::
 102
 103        cpucache_t *cc; /* this is per-CPU */
 104        preempt_disable();
 105        cc = cc_data(searchp);
 106        if (cc && cc->avail) {
 107                __free_block(searchp, cc_entry(cc), cc->avail);
 108                cc->avail = 0;
 109        }
 110        preempt_enable();
 111        return 0;
 112
 113Notice how the preemption statements must encompass every reference of the
 114critical variables.  Another example::
 115
 116        int buf[NR_CPUS];
 117        set_cpu_val(buf);
 118        if (buf[smp_processor_id()] == -1) printf(KERN_INFO "wee!\n");
 119        spin_lock(&buf_lock);
 120        /* ... */
 121
 122This code is not preempt-safe, but see how easily we can fix it by simply
 123moving the spin_lock up two lines.
 124
 125
 126Preventing preemption using interrupt disabling
 127===============================================
 128
 129
 130It is possible to prevent a preemption event using local_irq_disable and
 131local_irq_save.  Note, when doing so, you must be very careful to not cause
 132an event that would set need_resched and result in a preemption check.  When
 133in doubt, rely on locking or explicit preemption disabling.
 134
 135Note in 2.5 interrupt disabling is now only per-CPU (e.g. local).
 136
 137An additional concern is proper usage of local_irq_disable and local_irq_save.
 138These may be used to protect from preemption, however, on exit, if preemption
 139may be enabled, a test to see if preemption is required should be done.  If
 140these are called from the spin_lock and read/write lock macros, the right thing
 141is done.  They may also be called within a spin-lock protected region, however,
 142if they are ever called outside of this context, a test for preemption should
 143be made. Do note that calls from interrupt context or bottom half/ tasklets
 144are also protected by preemption locks and so may use the versions which do
 145not check preemption.
 146