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    - map_phys_fmr
26
27  which may not sleep and must be callable from any context.
28
29  The corresponding functions exported to upper level protocol
30  consumers:
31
32    - rdma_create_ah
33    - rdma_modify_ah
34    - rdma_query_ah
35    - rdma_destroy_ah
36    - ib_post_send
37    - ib_post_recv
38    - ib_req_notify_cq
39    - ib_map_phys_fmr
40
41  are therefore safe to call from any context.
42
43  In addition, the function
44
45    - ib_dispatch_event
46
47  used by low-level drivers to dispatch asynchronous events through
48  the midlayer is also safe to call from any context.
49
50Reentrancy
51----------
52
53  All of the methods in struct ib_device exported by a low-level
54  driver must be fully reentrant.  The low-level driver is required to
55  perform all synchronization necessary to maintain consistency, even
56  if multiple function calls using the same object are run
57  simultaneously.
58
59  The IB midlayer does not perform any serialization of function calls.
60
61  Because low-level drivers are reentrant, upper level protocol
62  consumers are not required to perform any serialization.  However,
63  some serialization may be required to get sensible results.  For
64  example, a consumer may safely call ib_poll_cq() on multiple CPUs
65  simultaneously.  However, the ordering of the work completion
66  information between different calls of ib_poll_cq() is not defined.
67
68Callbacks
69---------
70
71  A low-level driver must not perform a callback directly from the
72  same callchain as an ib_device method call.  For example, it is not
73  allowed for a low-level driver to call a consumer's completion event
74  handler directly from its post_send method.  Instead, the low-level
75  driver should defer this callback by, for example, scheduling a
76  tasklet to perform the callback.
77
78  The low-level driver is responsible for ensuring that multiple
79  completion event handlers for the same CQ are not called
80  simultaneously.  The driver must guarantee that only one CQ event
81  handler for a given CQ is running at a time.  In other words, the
82  following situation is not allowed::
83
84          CPU1                                    CPU2
85
86    low-level driver ->
87      consumer CQ event callback:
88        /* ... */
89        ib_req_notify_cq(cq, ...);
90                                          low-level driver ->
91        /* ... */                           consumer CQ event callback:
92                                              /* ... */
93        return from CQ event handler
94
95  The context in which completion event and asynchronous event
96  callbacks run is not defined.  Depending on the low-level driver, it
97  may be process context, softirq context, or interrupt context.
98  Upper level protocol consumers may not sleep in a callback.
99
100Hot-plug
101--------
102
103  A low-level driver announces that a device is ready for use by
104  consumers when it calls ib_register_device(), all initialization
105  must be complete before this call.  The device must remain usable
106  until the driver's call to ib_unregister_device() has returned.
107
108  A low-level driver must call ib_register_device() and
109  ib_unregister_device() from process context.  It must not hold any
110  semaphores that could cause deadlock if a consumer calls back into
111  the driver across these calls.
112
113  An upper level protocol consumer may begin using an IB device as
114  soon as the add method of its struct ib_client is called for that
115  device.  A consumer must finish all cleanup and free all resources
116  relating to a device before returning from the remove method.
117
118  A consumer is permitted to sleep in its add and remove methods.
119