Date: Fri, 27 Sep 2013 01:08:11 +0200
From: Andres Freund <andres@2ndquadrant.com>
To: Merlin Moncure <mmoncure@gmail.com>
Cc: =?utf-8?B?0JTQvNC40YLRgNC40Lkg0JTQtdCz0YLRj9GA0ZHQsg==?=
 <degtyaryov@gmail.com>,
 postgres performance list <pgsql-performance@postgresql.org>
Subject: Re: Cpu usage 100% on slave. s_lock problem.
Message-ID: <20130926230811.GA29658@awork2.anarazel.de>
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On 2013-08-27 12:17:55 -0500, Merlin Moncure wrote:
> On Tue, Aug 27, 2013 at 10:55 AM, Andres Freund <andres@2ndquadrant.com> wrote:
> > On 2013-08-27 09:57:38 -0500, Merlin Moncure wrote:
> >> + bool
> >> + RecoveryMightBeInProgress(void)
> >> + {
> >> +     /*
> >> +      * We check shared state each time only until we leave recovery mode. We
> >> +      * can't re-enter recovery, so there's no need to keep checking after the
> >> +      * shared variable has once been seen false.
> >> +      */
> >> +     if (!LocalRecoveryInProgress)
> >> +             return false;
> >> +     else
> >> +     {
> >> +             /* use volatile pointer to prevent code rearrangement */
> >> +             volatile XLogCtlData *xlogctl = XLogCtl;
> >> +
> >> +             /* Intentionally query xlogctl without spinlocking! */
> >> +             LocalRecoveryInProgress = xlogctl->SharedRecoveryInProgress;
> >> +
> >> +             return LocalRecoveryInProgress;
> >> +     }
> >> + }
> >
> > I don't think it's acceptable to *set* LocalRecoveryInProgress
> > here. That should only be done in the normal routine.
> 
> quite right -- that was a major error -- you could bypass the
> initialization call to the xlog with some bad luck.

I've seen this in profiles since, so I'd appreciate pushing this
forward.

Greetings,

Andres Freund

-- 
 Andres Freund	                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services


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