ovs-atomic: Use explicit memory order for ovs_refcount.

Use explicit variants of atomic operations for the ovs_refcount to
avoid the overhead of the default memory_order_seq_cst.

Adding a reference requires no memory ordering, as the calling thread
is already assumed to have protected access to the object being
reference counted.  Hence, memory_order_relaxed is used for
ovs_refcount_ref().  ovs_refcount_read() does not change the reference
count, so it can also use memory_order_relaxed.

Unreferencing an object needs a release barrier, so that none of the
accesses to the protected object are reordered after the atomic
decrement operation.  Additionally, an explicit acquire barrier is
needed before the object is recycled, to keep the subsequent accesses
to the object's memory from being reordered before the atomic
decrement operation.

This patch follows the memory ordering and argumentation discussed
here:

http://www.chaoticmind.net/~hcb/projects/boost.atomic/doc/atomic/usage_examples.html

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>

0 files changed, 0 insertions, 0 deletions