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Fix bug in the new wait-until-lwlock-is-free mechanism.

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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.
This commit is contained in:
Heikki Linnakangas 2012-01-30 20:56:35 +02:00
parent 82e83f46a2
commit 82d4b262d9
1 changed files with 16 additions and 9 deletions
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25
src/backend/storage/lmgr/lwlock.c
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@ -754,23 +754,31 @@ LWLockRelease(LWLockId lockid)
if (lock->exclusive == 0 && lock->shared == 0 && lock->releaseOK) if (lock->exclusive == 0 && lock->shared == 0 && lock->releaseOK)
{ {
/* /*
* Remove the to-be-awakened PGPROCs from the queue. If the front * Remove the to-be-awakened PGPROCs from the queue.
* 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).
*/ */
bool releaseOK = true; bool releaseOK = true;
proc = head; 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) if (proc->lwWaitMode != LW_EXCLUSIVE)
{ {
while (proc->lwWaitLink != NULL && while (proc->lwWaitLink != NULL &&
proc->lwWaitLink->lwWaitMode != LW_EXCLUSIVE) proc->lwWaitLink->lwWaitMode != LW_EXCLUSIVE)
{ {
proc = proc->lwWaitLink;
if (proc->lwWaitMode != LW_WAIT_UNTIL_FREE) if (proc->lwWaitMode != LW_WAIT_UNTIL_FREE)
releaseOK = false; releaseOK = false;
proc = proc->lwWaitLink;
} }
} }
/* proc is now the last PGPROC to be released */ /* proc is now the last PGPROC to be released */
@ -778,9 +786,8 @@ LWLockRelease(LWLockId lockid)
proc->lwWaitLink = NULL; proc->lwWaitLink = NULL;
/* /*
* Prevent additional wakeups until retryer gets to run. Backends * Prevent additional wakeups until retryer gets to run. Backends
* that are just waiting for the lock to become free don't prevent * that are just waiting for the lock to become free don't retry
* wakeups, because they might decide that they don't want the * automatically.
* lock, after all.
*/ */
if (proc->lwWaitMode != LW_WAIT_UNTIL_FREE) if (proc->lwWaitMode != LW_WAIT_UNTIL_FREE)
releaseOK = false; releaseOK = false;