LXR linux/Documentation/infiniband/core

   1===========================
   2InfiniBand Midlayer Locking
   3===========================
   4
   5  This guide is an attempt to make explicit the locking assumptions
   6  made by the InfiniBand midlayer.  It describes the requirements on
   7  both low-level drivers that sit below the midlayer and upper level
   8  protocols that use the midlayer.
   9
  10Sleeping and interrupt context
  11==============================
  12
  13  With the following exceptions, a low-level driver implementation of
  14  all of the methods in struct ib_device may sleep.  The exceptions
  15  are any methods from the list:
  16
  17    - create_ah
  18    - modify_ah
  19    - query_ah
  20    - destroy_ah
  21    - post_send
  22    - post_recv
  23    - poll_cq
  24    - req_notify_cq
  25
  26  which may not sleep and must be callable from any context.
  27
  28  The corresponding functions exported to upper level protocol
  29  consumers:
  30
  31    - rdma_create_ah
  32    - rdma_modify_ah
  33    - rdma_query_ah
  34    - rdma_destroy_ah
  35    - ib_post_send
  36    - ib_post_recv
  37    - ib_req_notify_cq
  38
  39  are therefore safe to call from any context.
  40
  41  In addition, the function
  42
  43    - ib_dispatch_event
  44
  45  used by low-level drivers to dispatch asynchronous events through
  46  the midlayer is also safe to call from any context.
  47
  48Reentrancy
  49----------
  50
  51  All of the methods in struct ib_device exported by a low-level
  52  driver must be fully reentrant.  The low-level driver is required to
  53  perform all synchronization necessary to maintain consistency, even
  54  if multiple function calls using the same object are run
  55  simultaneously.
  56
  57  The IB midlayer does not perform any serialization of function calls.
  58
  59  Because low-level drivers are reentrant, upper level protocol
  60  consumers are not required to perform any serialization.  However,
  61  some serialization may be required to get sensible results.  For
  62  example, a consumer may safely call ib_poll_cq() on multiple CPUs
  63  simultaneously.  However, the ordering of the work completion
  64  information between different calls of ib_poll_cq() is not defined.
  65
  66Callbacks
  67---------
  68
  69  A low-level driver must not perform a callback directly from the
  70  same callchain as an ib_device method call.  For example, it is not
  71  allowed for a low-level driver to call a consumer's completion event
  72  handler directly from its post_send method.  Instead, the low-level
  73  driver should defer this callback by, for example, scheduling a
  74  tasklet to perform the callback.
  75
  76  The low-level driver is responsible for ensuring that multiple
  77  completion event handlers for the same CQ are not called
  78  simultaneously.  The driver must guarantee that only one CQ event
  79  handler for a given CQ is running at a time.  In other words, the
  80  following situation is not allowed::
  81
  82          CPU1                                    CPU2
  83
  84    low-level driver ->
  85      consumer CQ event callback:
  86        /* ... */
  87        ib_req_notify_cq(cq, ...);
  88                                          low-level driver ->
  89        /* ... */                           consumer CQ event callback:
  90                                              /* ... */
  91        return from CQ event handler
  92
  93  The context in which completion event and asynchronous event
  94  callbacks run is not defined.  Depending on the low-level driver, it
  95  may be process context, softirq context, or interrupt context.
  96  Upper level protocol consumers may not sleep in a callback.
  97
  98Hot-plug
  99--------
 100
 101  A low-level driver announces that a device is ready for use by
 102  consumers when it calls ib_register_device(), all initialization
 103  must be complete before this call.  The device must remain usable
 104  until the driver's call to ib_unregister_device() has returned.
 105
 106  A low-level driver must call ib_register_device() and
 107  ib_unregister_device() from process context.  It must not hold any
 108  semaphores that could cause deadlock if a consumer calls back into
 109  the driver across these calls.
 110
 111  An upper level protocol consumer may begin using an IB device as
 112  soon as the add method of its struct ib_client is called for that
 113  device.  A consumer must finish all cleanup and free all resources
 114  relating to a device before returning from the remove method.
 115
 116  A consumer is permitted to sleep in its add and remove methods.
 117