Andres Freund <> wrote:
> On 2015-02-14 17:25:00 +0000, Kevin Grittner wrote:

>>> I think we should simply move the
>>>  buf->flags &= ~BM_PIN_COUNT_WAITER (Inside LockBuffer)
>> I think you meant inside UnpinBuffer?
> No, LockBufferHdr. What I meant was that the pincount can only be
> manipulated while the buffer header spinlock is held.

Oh, I see what you were saying -- I had read that a different way
entirely.  Got it.

>> Even though it appears to be a long-standing bug, there don't
>> appear to have been any field reports, so it doesn't seem like
>> something to back-patch.
> I was wondering about that as well. But I don't think I agree.
> The most likely scenario for this to fail is in full table
> vacuums that have to freeze rows - those are primarily triggered
> by autovacuum. I don't think it's likely that such a error
> message would be discovered in the logs unless it happens very
> regularly.

I guess we have some time before the next minor release to find any
problems with this; perhaps the benefit would outweigh the risk.
Anyone else want to weigh in on that?

> You can't manipulate flags without holding the spinlock.
> Otherwise you (or the other writer) can easily cancel the other
> sides effects.

So is the attached more like what you had in mind?  If not, feel
free to post a patch.  :-)

Kevin Grittner
The Enterprise PostgreSQL Company
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index e1e6240..6640172 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -1548,7 +1548,6 @@ UnpinBuffer(volatile BufferDesc *buf, bool fixOwner)
 			/* we just released the last pin other than the waiter's */
 			int			wait_backend_pid = buf->wait_backend_pid;
-			buf->flags &= ~BM_PIN_COUNT_WAITER;
@@ -3273,6 +3272,13 @@ LockBufferForCleanup(Buffer buffer)
+		/*
+		 * Clear the flag unconditionally here, because otherwise a spurious
+		 * signal (which is allowed) could make it look like an error.
+		 */
+		LockBufHdr(bufHdr);
+		bufHdr->flags &= ~BM_PIN_COUNT_WAITER;
+		UnlockBufHdr(bufHdr);
 		PinCountWaitBuf = NULL;
 		/* Loop back and try again */
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