From 82d4b262d9614958da38003cdc9d56915d9bcda0 Mon Sep 17 00:00:00 2001
From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: Mon, 30 Jan 2012 20:56:35 +0200
Subject: [PATCH] Fix bug in the new wait-until-lwlock-is-free mechanism.
If there was a wait-until-free process in the head of the wait queue,
followed by an exclusive locker, the exclusive locker was not be woken up
as it should.
---
src/backend/storage/lmgr/lwlock.c | 25 ++++++++++++++++---------
1 file changed, 16 insertions(+), 9 deletions(-)
diff --git a/src/backend/storage/lmgr/lwlock.c b/src/backend/storage/lmgr/lwlock.c
index bee35b8c1cf..6511faf985d 100644
--- a/src/backend/storage/lmgr/lwlock.c
+++ b/src/backend/storage/lmgr/lwlock.c
@@ -754,23 +754,31 @@ LWLockRelease(LWLockId lockid)
if (lock->exclusive == 0 && lock->shared == 0 && lock->releaseOK)
{
/*
- * Remove the to-be-awakened PGPROCs from the queue. If the front
- * waiter wants exclusive lock, awaken him only. Otherwise awaken
- * as many waiters as want shared access (or just want to be
- * woken up when the lock becomes free without acquiring it,
- * ie. LWLockWaitUntilFree).
+ * Remove the to-be-awakened PGPROCs from the queue.
*/
bool releaseOK = true;
proc = head;
+
+ /*
+ * First wake up any backends that want to be woken up without
+ * acquiring the lock.
+ */
+ while (proc->lwWaitMode == LW_WAIT_UNTIL_FREE && proc->lwWaitLink)
+ proc = proc->lwWaitLink;
+
+ /*
+ * If the front waiter wants exclusive lock, awaken him only.
+ * Otherwise awaken as many waiters as want shared access.
+ */
if (proc->lwWaitMode != LW_EXCLUSIVE)
{
while (proc->lwWaitLink != NULL &&
proc->lwWaitLink->lwWaitMode != LW_EXCLUSIVE)
{
- proc = proc->lwWaitLink;
if (proc->lwWaitMode != LW_WAIT_UNTIL_FREE)
releaseOK = false;
+ proc = proc->lwWaitLink;
}
}
/* proc is now the last PGPROC to be released */
@@ -778,9 +786,8 @@ LWLockRelease(LWLockId lockid)
proc->lwWaitLink = NULL;
/*
* Prevent additional wakeups until retryer gets to run. Backends
- * that are just waiting for the lock to become free don't prevent
- * wakeups, because they might decide that they don't want the
- * lock, after all.
+ * that are just waiting for the lock to become free don't retry
+ * automatically.
*/
if (proc->lwWaitMode != LW_WAIT_UNTIL_FREE)
releaseOK = false;
--
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