From: Robert Haas <rhaas@postgresql.org>
Date: Sat, 7 May 2011 01:53:55 +0000 (-0400)
Subject: Add comment about memory reordering to PredicateLockTupleRowVersionLink.
X-Git-Tag: REL9_1_BETA2~157
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=71932ecc2b6ca0d748176a7e8b11d3575bf4caf3;p=thirdparty%2Fpostgresql.git

Add comment about memory reordering to PredicateLockTupleRowVersionLink.

Dan Ports, per head-scratching from Simon Riggs and myself.
---

diff --git a/src/backend/storage/lmgr/predicate.c b/src/backend/storage/lmgr/predicate.c
index 48ff9cc151d..3b3158efe55 100644
--- a/src/backend/storage/lmgr/predicate.c
+++ b/src/backend/storage/lmgr/predicate.c
@@ -2283,6 +2283,9 @@ PredicateLockTupleRowVersionLink(const Relation relation,
 	 * locks. Even if a serializable transaction starts concurrently,
 	 * we know it can't take any SIREAD locks on the modified tuple
 	 * because the caller is holding the associated buffer page lock.
+	 * Memory reordering isn't an issue; the memory barrier in the
+	 * LWLock acquisition guarantees that this read occurs while the
+	 * buffer page lock is held.
 	 */
 	if (!TransactionIdIsValid(PredXact->SxactGlobalXmin))
 		return;