Documentation/infiniband/core_locking.txt

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