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Commit82d4b26

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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 upas it should.
1 parent82e83f4 commit82d4b26

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‎src/backend/storage/lmgr/lwlock.c

Lines changed: 16 additions & 9 deletions
Original file line numberDiff line numberDiff line change
@@ -754,33 +754,40 @@ LWLockRelease(LWLockId lockid)
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if (lock->exclusive==0&&lock->shared==0&&lock->releaseOK)
755755
{
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/*
757-
* Remove the to-be-awakened PGPROCs from the queue. If the front
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* waiter wants exclusive lock, awaken him only. Otherwise awaken
759-
* as many waiters as want shared access (or just want to be
760-
* woken up when the lock becomes free without acquiring it,
761-
* ie. LWLockWaitUntilFree).
757+
* Remove the to-be-awakened PGPROCs from the queue.
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*/
763759
boolreleaseOK= true;
764760

765761
proc=head;
762+
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/*
764+
* First wake up any backends that want to be woken up without
765+
* acquiring the lock.
766+
*/
767+
while (proc->lwWaitMode==LW_WAIT_UNTIL_FREE&&proc->lwWaitLink)
768+
proc=proc->lwWaitLink;
769+
770+
/*
771+
* If the front waiter wants exclusive lock, awaken him only.
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* Otherwise awaken as many waiters as want shared access.
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*/
766774
if (proc->lwWaitMode!=LW_EXCLUSIVE)
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{
768776
while (proc->lwWaitLink!=NULL&&
769777
proc->lwWaitLink->lwWaitMode!=LW_EXCLUSIVE)
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{
771-
proc=proc->lwWaitLink;
772779
if (proc->lwWaitMode!=LW_WAIT_UNTIL_FREE)
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releaseOK= false;
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proc=proc->lwWaitLink;
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}
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}
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/* proc is now the last PGPROC to be released */
777785
lock->head=proc->lwWaitLink;
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proc->lwWaitLink=NULL;
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/*
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* Prevent additional wakeups until retryer gets to run. Backends
781-
* that are just waiting for the lock to become free don't prevent
782-
* wakeups, because they might decide that they don't want the
783-
* lock, after all.
789+
* that are just waiting for the lock to become free don't retry
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* automatically.
784791
*/
785792
if (proc->lwWaitMode!=LW_WAIT_UNTIL_FREE)
786793
releaseOK= false;

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