From bc208a5a2f9eb34bf795374fa5381e0c82073255 Mon Sep 17 00:00:00 2001 From: Andres Freund Date: Mon, 23 Feb 2015 16:11:11 +0100 Subject: [PATCH] Guard against spurious signals in LockBufferForCleanup. When LockBufferForCleanup() has to wait for getting a cleanup lock on a buffer it does so by setting a flag in the buffer header and then wait for other backends to signal it using ProcWaitForSignal(). Unfortunately LockBufferForCleanup() missed that ProcWaitForSignal() can return for other reasons than the signal it is hoping for. If such a spurious signal arrives the wait flags on the buffer header will still be set. That then triggers "ERROR: multiple backends attempting to wait for pincount 1". The fix is simple, unset the flag if still set when retrying. That implies an additional spinlock acquisition/release, but that's unlikely to matter given the cost of waiting for a cleanup lock. Alternatively it'd have been possible to move responsibility for maintaining the relevant flag to the waiter all together, but that might have had negative consequences due to possible floods of signals. Besides being more invasive. This looks to be a very longstanding bug. The relevant code in LockBufferForCleanup() hasn't changed materially since its introduction and ProcWaitForSignal() was documented to return for unrelated reasons since 8.2. The master only patch series removing ImmediateInterruptOK made it much easier to hit though, as ProcSendSignal/ProcWaitForSignal now uses a latch shared with other tasks. Per discussion with Kevin Grittner, Tom Lane and me. Backpatch to all supported branches. Discussion: 11553.1423805224@sss.pgh.pa.us --- src/backend/storage/buffer/bufmgr.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+) diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c index e1e6240fe3..a68eae8169 100644 --- a/src/backend/storage/buffer/bufmgr.c +++ b/src/backend/storage/buffer/bufmgr.c @@ -3273,6 +3273,20 @@ LockBufferForCleanup(Buffer buffer) else ProcWaitForSignal(); + /* + * Remove flag marking us as waiter. Normally this will not be set + * anymore, but ProcWaitForSignal() can return for other signals as + * well. We take care to only reset the flag if we're the waiter, as + * theoretically another backend could have started waiting. That's + * impossible with the current usages due to table level locking, but + * better be safe. + */ + LockBufHdr(bufHdr); + if ((bufHdr->flags & BM_PIN_COUNT_WAITER) != 0 && + bufHdr->wait_backend_pid == MyProcPid) + bufHdr->flags &= ~BM_PIN_COUNT_WAITER; + UnlockBufHdr(bufHdr); + PinCountWaitBuf = NULL; /* Loop back and try again */ } -- 2.40.0