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 bind_mw
19 post_send
20 post_recv
21 poll_cq
22 req_notify_cq
23 map_phys_fmr
24
25 which may not sleep and must be callable from any context.
26
27 The corresponding functions exported to upper level protocol
28 consumers:
29
30 ib_create_ah
31 ib_modify_ah
32 ib_query_ah
33 ib_destroy_ah
34 ib_bind_mw
35 ib_post_send
36 ib_post_recv
37 ib_req_notify_cq
38 ib_map_phys_fmr
39
40 are therefore safe to call from any context.
41
42 In addition, the function
43
44 ib_dispatch_event
45
46 used by low-level drivers to dispatch asynchronous events through
47 the midlayer is also safe to call from any context.
48
49 Reentrancy
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
66 Callbacks
67
68 A low-level driver must not perform a callback directly from the
69 same callchain as an ib_device method call. For example, it is not
70 allowed for a low-level driver to call a consumer's completion event
71 handler directly from its post_send method. Instead, the low-level
72 driver should defer this callback by, for example, scheduling a
73 tasklet to perform the callback.
74
75 The low-level driver is responsible for ensuring that multiple
76 completion event handlers for the same CQ are not called
77 simultaneously. The driver must guarantee that only one CQ event
78 handler for a given CQ is running at a time. In other words, the
79 following situation is not allowed:
80
81 CPU1 CPU2
82
83 low-level driver ->
84 consumer CQ event callback:
85 /* ... */
86 ib_req_notify_cq(cq, ...);
87 low-level driver ->
88 /* ... */ consumer CQ event callback:
89 /* ... */
90 return from CQ event handler
91
92 The context in which completion event and asynchronous event
93 callbacks run is not defined. Depending on the low-level driver, it
94 may be process context, softirq context, or interrupt context.
95 Upper level protocol consumers may not sleep in a callback.
96
97 Hot-plug
98
99 A low-level driver announces that a device is ready for use by
100 consumers when it calls ib_register_device(), all initialization
101 must be complete before this call. The device must remain usable
102 until the driver's call to ib_unregister_device() has returned.
103
104 A low-level driver must call ib_register_device() and
105 ib_unregister_device() from process context. It must not hold any
106 semaphores that could cause deadlock if a consumer calls back into
107 the driver across these calls.
108
109 An upper level protocol consumer may begin using an IB device as
110 soon as the add method of its struct ib_client is called for that
111 device. A consumer must finish all cleanup and free all resources
112 relating to a device before returning from the remove method.
113
114 A consumer is permitted to sleep in its add and remove methods.
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