"Qingqing Zhou" <[EMAIL PROTECTED]> writes: > ... However, a possible execution sequence involving another process > doing UnpinBuffer() may look like this:
> unpinner: lockHdr(); read and reset flag; unlockHdr(); > waiter: lockHdr(); reset flag; unlockHdr(); ProcCancelWaitForSignal(); > unpinner: ProcSendSignal(); Hmm ... I remember having convinced myself this code was OK, but I guess I was wrong. > After this, the proc->sem will be bumped to 1 unexpectedly ... Since this > problem is rare, a possible fix is to put a critical section around line 1 > to 7 and remove UnlockBuffers() accordingly. No, that would make any attempt to control-C a VACUUM have a significant probability for panicking the whole database. I think a better fix might be to arrange for an extra PGSemaphoreUnlock to not be a problem. This is already true in lwlock.c, and in the pin-count-waiter too (it'll just cause an extra cycle around the loop). We'd have to modify ProcSleep to loop until it sees that someone has actually granted or denied the lock, but that does not seem too hard. (First thought about it is to change MyProc->waitStatus to have three states, "waiting/ok/denied".) ProcCancelWaitForSignal could then go away entirely. regards, tom lane ---------------------------(end of broadcast)--------------------------- TIP 2: Don't 'kill -9' the postmaster