Re: Memory ordering issue in LWLockRelease, WakeupWaiters, WALInsertSlotRelease

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.

--
Robert Haas
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