Andres Freund <and...@2ndquadrant.com> wrote: > I don't think it's actually 675333 at fault here. I think it's a > long standing bug in LockBufferForCleanup() that can just much > easier be hit with the new interrupt code.
The patches I'll be posting soon make it even easier to hit, which is why I was trying to sort this out when Tom noticed the buildfarm issues. > Imagine what happens in LockBufferForCleanup() when > ProcWaitForSignal() returns spuriously - something it's > documented to possibly do (and which got more likely with the new > patches). In the normal case UnpinBuffer() will have unset > BM_PIN_COUNT_WAITER - but in a spurious return it'll still be set > and LockBufferForCleanup() will see it still set. That analysis makes sense to me. > I think we should simply move the > buf->flags &= ~BM_PIN_COUNT_WAITER (Inside LockBuffer) I think you meant inside UnpinBuffer? > to LockBufferForCleanup, besides the PinCountWaitBuf = NULL. > Afaics, that should do the trick. I tried that on the master branch (33e879c) (attached) and it passes `make check-world` with no problems. I'm reviewing the places that BM_PIN_COUNT_WAITER appears, to see if I can spot any flaw in this. Does anyone else see a problem with 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. -- Kevin Grittner EDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c index e1e6240..40b2194 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; UnlockBufHdr(buf); ProcSendSignal(wait_backend_pid); } @@ -3273,6 +3272,7 @@ LockBufferForCleanup(Buffer buffer) else ProcWaitForSignal(); + bufHdr->flags &= ~BM_PIN_COUNT_WAITER; PinCountWaitBuf = NULL; /* Loop back and try again */ }
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