===========================
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:
- create_ah
- modify_ah
- query_ah
- destroy_ah
- post_send
- post_recv
- poll_cq
- req_notify_cq
- map_phys_fmr
which may not sleep and must be callable from any context.
The corresponding functions exported to upper level protocol
consumers:
- rdma_create_ah
- rdma_modify_ah
- rdma_query_ah
- rdma_destroy_ah
- ib_post_send
- ib_post_recv
- ib_req_notify_cq
- ib_map_phys_fmr
are therefore safe to call from any context.
In addition, the function
- ib_dispatch_event
used by low-level drivers to dispatch asynchronous events through
the midlayer is also safe to call from any context.
Reentrancy
----------
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
simultaneously.
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.
Callbacks
---------
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.
Hot-plug
--------
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.