IB/hfi1: Eliminate synchronize_rcu() in mr delete
authorMike Marciniszyn <mike.marciniszyn@intel.com>
Tue, 21 Mar 2017 00:26:26 +0000 (17:26 -0700)
committerDoug Ledford <dledford@redhat.com>
Wed, 5 Apr 2017 18:45:09 +0000 (14:45 -0400)
commitb58fc8049790ca780fb42bd2faabe9b921aedf44
treeac50f185872ee8996710f6f0a57c91e2be4e76f6
parent243d9f436f89f95c304011bd32485afc27581986
IB/hfi1: Eliminate synchronize_rcu() in mr delete

The synchronize_rcu() call can be eliminated to improve memory deregistration
performance.

There are two key fields involved:
- The rcu pointer itself
- the lkey_published field

To close the window between the rcu read of the mregion pointer and the
reference count the code should:

1. To lkey/rkey validation (reader)

Read the rcu pointer.  If the pointer is non-NULL, get a reference.

To the current validation tests use a READ_ONCE() on the lkey_published.

Upon any failure release the reference.

2. To the remove logic (delete)

Insure the published is zeroed prior to setting the pointer to NULL.
This requires using rcu_assign_pointer() to insure lkey_published
is written prior to the NULL.

3. To the insert logic (add)

Insure the published is set use an rcu_assign_pointer() to insure the
pointer is after all MR fields.

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>
drivers/infiniband/sw/rdmavt/mr.c