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