Re: [PERFORM] Cpu usage 100% on slave. s_lock problem.

On Wed, Oct 2, 2013 at 9:45 AM, Ants Aasma <ants(at)cybertec(dot)at> wrote:
> I haven't reviewed the code in as much detail to say if there is an
> actual race here, I tend to think there's probably not, but the
> specific pattern that I had in mind is that with the following actual
> code:

hm. I think there *is* a race. 2+ threads could race to the line:

LocalRecoveryInProgress = xlogctl->SharedRecoveryInProgress;

and simultaneously set the value of LocalRecoveryInProgress to false,
and both engage InitXLOGAccess, which is destructive. The move
operation is atomic, but I don't think there's any guarantee the reads
to xlogctl->SharedRecoveryInProgress are ordered between threads
without a lock.

I don't think the memory barrier will fix this. Do you agree? If so,
my earlier patch (recovery4) is another take on this problem, and
arguable safer; the unlocked read is in a separate path that does not
engage InitXLOGAccess()

merlin