T589: Fix insufficient ordering guarantees in bakery lock

bakery_lock_get() uses DMB LD after lock acquisition and
bakery_lock_release() uses DMB ST before releasing the lock. This is
insufficient in both cases. With just DMB LD, stores in the critical
section can be reordered before the DMB LD which could mean writes in
the critical section completing before the lock has been acquired
successfully. Similarly, with just DMB ST, a load in the critical section
could be reordered after the the DMB ST. DMB is the least expensive
barrier that can provide the required ordering.

Signed-off-by: Raghu Krishnamurthy <raghu.ncstate@icloud.com>
Change-Id: Ieb74cbf5b76b09e1789331b71f37f7c660221b0e
2 files changed