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Commitd699ba4

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Fix WakeupWaiters() to not wake up an exclusive locker unnecessarily.
WakeupWaiters() is supposed to wake up all LW_WAIT_UNTIL_FREE waiters ofthe slot, but the loop incorrectly also woke up the first LW_EXCLUSIVEwaiter, if there was no LW_WAIT_UNTIL_FREE waiters in the queue.Noted by Andres Freund. This code is new in 9.4, so no backpatching.
1 parent6c2744f commitd699ba4

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  • src/backend/access/transam

1 file changed

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‎src/backend/access/transam/xlog.c

Lines changed: 5 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -1842,15 +1842,14 @@ WakeupWaiters(XLogRecPtr EndPos)
18421842
slot->xlogInsertingAt=EndPos;
18431843

18441844
/*
1845-
* See if there are any waiters that need to be woken up.
1845+
* See if there are any LW_WAIT_UNTIL_FREE waiters that need to be woken
1846+
* up. They are always in the front of the queue.
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*/
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head=slot->head;
18481849

1849-
if (head!=NULL)
1850+
if (head!=NULL&&head->lwWaitMode==LW_WAIT_UNTIL_FREE)
18501851
{
18511852
proc=head;
1852-
1853-
/* LW_WAIT_UNTIL_FREE waiters are always in the front of the queue */
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next=proc->lwWaitLink;
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while (next&&next->lwWaitMode==LW_WAIT_UNTIL_FREE)
18561855
{
@@ -1862,6 +1861,8 @@ WakeupWaiters(XLogRecPtr EndPos)
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slot->head=next;
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proc->lwWaitLink=NULL;
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}
1864+
else
1865+
head=NULL;
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/* We are done updating shared state of the lock itself. */
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SpinLockRelease(&slot->mutex);

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