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.