tree 1a0757409c89c6236d9c0ffa1eb0f5a63faa2527
parent d227fa7288adebe5ba37fa8e4a589c977d4e4a34
author Olaf Kirch <okir@lst.de> 1209502013 -0700
committer Roland Dreier <rolandd@cisco.com> 1209502013 -0700

mlx4_core: Avoid recycling old FMR R_Keys too soon

When a FMR is unmapped, mlx4 resets the map count to 0, and clears the
upper part of the R_Key which is used as the sequence counter.

This poses a problem for RDS, which uses ib_fmr_unmap as a fence
operation.  RDS assumes that after issuing an unmap, the old R_Keys
will be invalid for a "reasonable" period of time. For instance,
Oracle processes uses shared memory buffers allocated from a pool of
buffers.  When a process dies, we want to reclaim these buffers -- but
we must make sure there are no pending RDMA operations to/from those
buffers.  The only way to achieve that is by using unmap and sync the
TPT.

However, when the sequence count is reset on unmap, there is a high
likelihood that a new mapping will be given the same R_Key that was
issued a few milliseconds ago.

To prevent this, don't reset the sequence count when unmapping a FMR.

Signed-off-by: Olaf Kirch <olaf.kirch@oracle.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>