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Documentation / infiniband / core_locking.txt




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Based on kernel version 4.7.2. Page generated on 2016-08-22 22:46 EST.

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.
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