Add barriers to the latch code.

Since their introduction latches have required barriers in SetLatch
and ResetLatch - but when they were introduced there wasn't any
barrier abstraction. Instead latches were documented to rely on the
callsites to provide barrier semantics.

Now that the barrier support looks halfway complete, add the necessary
barriers to both latch implementations.

Also remove a now superflous lock acquisition from syncrep.c and a
superflous (and insufficient) barrier from freelist.c. There might be
other cases that can now be simplified, but those are the only ones
I've seen on a quick scan.

We might want to backpatch this at some later point, but right now the
barrier infrastructure in the backbranches isn't totally on par with
master.

Discussion: 20150112154026.GB2092@awork2.anarazel.de

1 files changed, 0 insertions, 10 deletions

diff --git a/src/backend/replication/syncrep.c b/src/backend/replication/syncrep.c
index c99c270a7ca..b2b3a81f54c 100644
--- a/src/backend/replication/syncrep.c
+++ b/src/backend/replication/syncrep.c
@@ -172,20 +172,10 @@ SyncRepWaitForLSN(XLogRecPtr XactCommitLSN)
 		 * walsender changes the state to SYNC_REP_WAIT_COMPLETE, it will
 		 * never update it again, so we can't be seeing a stale value in that
 		 * case.
-		 *
-		 * Note: on machines with weak memory ordering, the acquisition of the
-		 * lock is essential to avoid race conditions: we cannot be sure the
-		 * sender's state update has reached main memory until we acquire the
-		 * lock.  We could get rid of this dance if SetLatch/ResetLatch
-		 * contained memory barriers.
 		 */
 		syncRepState = MyProc->syncRepState;
 		if (syncRepState == SYNC_REP_WAITING)
-		{
-			LWLockAcquire(SyncRepLock, LW_SHARED);
 			syncRepState = MyProc->syncRepState;
-			LWLockRelease(SyncRepLock);
-		}
 		if (syncRepState == SYNC_REP_WAIT_COMPLETE)
 			break;