Re: Memory ordering issue in LWLockRelease, WakeupWaiters, WALInsertSlotRelease

On 2014-02-17 13:49:01 -0500, Robert Haas wrote:
> On Sat, Feb 15, 2014 at 11:17 AM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> > On 2014-02-15 16:18:00 +0100, Andres Freund wrote:
> >> On 2014-02-15 10:06:41 -0500, Tom Lane wrote:
> >> > Andres Freund <andres(at)2ndquadrant(dot)com> writes:
> >> > > My current conclusion is that backporting barriers.h is by far the most
> >> > > reasonable way to go. The compiler problems have been ironed out by
> >> > > now...
> >> >
> >> > -1. IMO that code is still quite unproven, and what's more, the
> >> > problem we're discussing here is completely hypothetical. If it
> >> > were real, we'd have field evidence of it. We've not had that
> >> > much trouble seeing instances of even very narrow race-condition
> >> > windows in the past.
> >>
> >> Well, the problem is that few of us have access to interesting !x86
> >> machines to run tests, and that's where we'd see problems (since x86
> >> gives enough guarantees to avoid this unless the compiler reorders
> >> stuff). I am personally fine with just using volatiles to avoid
> >> reordering in the older branches, but Florian argued against it.
> >
> > Here's patches doing that. The 9.3 version also applies to 9.2; the 9.1
> > version applies back to 8.4.
>
> I have no confidence that this isn't going to be real bad for performance.

It's just a write barrier which evaluates to a pure compiler barrier on
x86 anyway?
And it's in a loop that's only entered when the kernel is entered anyway
to wake up the other backend.

What should that affect significantly?

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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