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+INFINIBAND MIDLAYER LOCKING
+ This guide is an attempt to make explicit the locking assumptions
+ made by the InfiniBand midlayer. It describes the requirements on
+ both low-level drivers that sit below the midlayer and upper level
+ protocols that use the midlayer.
+Sleeping and interrupt context
+ With the following exceptions, a low-level driver implementation of
+ all of the methods in struct ib_device may sleep. The exceptions
+ are any methods from the list:
+ which may not sleep and must be callable from any context.
+ The corresponding functions exported to upper level protocol
+ are therefore safe to call from any context.
+ In addition, the function
+ used by low-level drivers to dispatch asynchronous events through
+ the midlayer is also safe to call from any context.
+ All of the methods in struct ib_device exported by a low-level
+ driver must be fully reentrant. The low-level driver is required to
+ perform all synchronization necessary to maintain consistency, even
+ if multiple function calls using the same object are run
+ The IB midlayer does not perform any serialization of function calls.
+ Because low-level drivers are reentrant, upper level protocol
+ consumers are not required to perform any serialization. However,
+ some serialization may be required to get sensible results. For
+ example, a consumer may safely call ib_poll_cq() on multiple CPUs
+ simultaneously. However, the ordering of the work completion
+ information between different calls of ib_poll_cq() is not defined.
+ A low-level driver must not perform a callback directly from the
+ same callchain as an ib_device method call. For example, it is not
+ allowed for a low-level driver to call a consumer's completion event
+ handler directly from its post_send method. Instead, the low-level
+ driver should defer this callback by, for example, scheduling a
+ tasklet to perform the callback.
+ The low-level driver is responsible for ensuring that multiple
+ completion event handlers for the same CQ are not called
+ simultaneously. The driver must guarantee that only one CQ event
+ handler for a given CQ is running at a time. In other words, the
+ following situation is not allowed:
+ CPU1 CPU2
+ low-level driver ->
+ consumer CQ event callback:
+ /* ... */
+ ib_req_notify_cq(cq, ...);
+ low-level driver ->
+ /* ... */ consumer CQ event callback:
+ /* ... */
+ return from CQ event handler
+ The context in which completion event and asynchronous event
+ callbacks run is not defined. Depending on the low-level driver, it
+ may be process context, softirq context, or interrupt context.
+ Upper level protocol consumers may not sleep in a callback.
+ A low-level driver announces that a device is ready for use by
+ consumers when it calls ib_register_device(), all initialization
+ must be complete before this call. The device must remain usable
+ until the driver's call to ib_unregister_device() has returned.
+ A low-level driver must call ib_register_device() and
+ ib_unregister_device() from process context. It must not hold any
+ semaphores that could cause deadlock if a consumer calls back into
+ the driver across these calls.
+ An upper level protocol consumer may begin using an IB device as
+ soon as the add method of its struct ib_client is called for that
+ device. A consumer must finish all cleanup and free all resources
+ relating to a device before returning from the remove method.
+ A consumer is permitted to sleep in its add and remove methods.