From: | Andres Freund <andres(at)2ndquadrant(dot)com> |
---|---|
To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Florian Pflug <fgp(at)phlo(dot)org>, PostgreSQL-development Hackers <pgsql-hackers(at)postgresql(dot)org>, Heikki Linnakangas <hlinnakangas(at)vmware(dot)com> |
Subject: | Re: Memory ordering issue in LWLockRelease, WakeupWaiters, WALInsertSlotRelease |
Date: | 2014-02-17 18:55:19 |
Message-ID: | 20140217185519.GE7161@awork2.anarazel.de |
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Thread: | |
Lists: | pgsql-hackers |
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/
PostgreSQL Development, 24x7 Support, Training & Services
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