From 92f4786fa9b730fd12cbfe973eb96addc6e98924 Mon Sep 17 00:00:00 2001
From: Simon Riggs <simon@2ndQuadrant.com>
Date: Sat, 26 Mar 2011 10:09:37 +0000
Subject: [PATCH] Additional test for each commit in sync rep path to plug
minute possibility of race condition that would effect performance only.
Requested by Robert Haas. Re-arrange related comments.
---
src/backend/replication/syncrep.c | 25 ++++++++++++++++---------
1 file changed, 16 insertions(+), 9 deletions(-)
diff --git a/src/backend/replication/syncrep.c b/src/backend/replication/syncrep.c
index e99b43d8e0d..17c255480eb 100644
--- a/src/backend/replication/syncrep.c
+++ b/src/backend/replication/syncrep.c
@@ -114,21 +114,28 @@ SyncRepWaitForLSN(XLogRecPtr XactCommitLSN)
/* Reset the latch before adding ourselves to the queue. */
ResetLatch(&MyProc->waitLatch);
- /*
- * Set our waitLSN so WALSender will know when to wake us, and add
- * ourselves to the queue.
- */
LWLockAcquire(SyncRepLock, LW_EXCLUSIVE);
Assert(MyProc->syncRepState == SYNC_REP_NOT_WAITING);
- if (!WalSndCtl->sync_standbys_defined)
+
+ /*
+ * We don't wait for sync rep if WalSndCtl->sync_standbys_defined is
+ * not set. See SyncRepUpdateSyncStandbysDefined.
+ *
+ * Also check that the standby hasn't already replied. Unlikely
+ * race condition but we'll be fetching that cache line anyway
+ * so its likely to be a low cost check.
+ */
+ if (!WalSndCtl->sync_standbys_defined ||
+ XLByteLE(XactCommitLSN, WalSndCtl->lsn))
{
- /*
- * We don't wait for sync rep if WalSndCtl->sync_standbys_defined is
- * not set. See SyncRepUpdateSyncStandbysDefined.
- */
LWLockRelease(SyncRepLock);
return;
}
+
+ /*
+ * Set our waitLSN so WALSender will know when to wake us, and add
+ * ourselves to the queue.
+ */
MyProc->waitLSN = XactCommitLSN;
MyProc->syncRepState = SYNC_REP_WAITING;
SyncRepQueueInsert();
--
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