Re: Memory ordering issue in LWLockRelease, WakeupWaiters, WALInsertSlotRelease

From: "Andres Freund" <andres(at)2ndquadrant(dot)com>
> which means they manipulate the lwWaitLink queue without
> protection. That's done intentionally. The code tries to protect against
> corruption of the list to do a woken up backend acquiring a lock (this
> or an independent one) by only continuing when the lwWaiting flag is set
> to false. Unfortunately there's absolutely no guarantee that a) the
> assignment to lwWaitLink and lwWaiting are done in that order b) that
> the stores are done in-order from the POV of other backends.
> So what we need to do is to acquire a write barrier between the
> assignments to lwWaitLink and lwWaiting, i.e.
> proc->lwWaitLink = NULL;
> pg_write_barrier();
> proc->lwWaiting = false;
> the reader side already uses an implicit barrier by using spinlocks.

I've got a report from one customer that they encountered a hang during
performance benchmarking. They were using PostgreSQL 9.2.4. I remember
that the stack trace showed many backends blocked forever at LWLockAcuuire()
during btree insert operation. I'm not sure this has something to do with
what you are raising, but the release notes for 9.2.5/6 doesn't suggest any
fixes for this. So I felt there is something wrong with lwlocks.

Do you think that your question could cause my customer's problem --
backends block at lwlock forever?

Regards
MauMau