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

On 2013-11-21 16:25:02 +0200, Heikki Linnakangas wrote:
> Hmm. All callers of RecoveryInProgress() must be prepared to handle the case
> that RecoveryInProgress() returns true, but the system is no longer in
> recovery. No matter what locking we do in RecoveryInProgress(), the startup
> process might finish recovery just after RecoveryInProgress() has returned.

True.

> What about the attached? It reads the shared variable without a lock or
> barrier. If it returns 'true', but the system in fact just exited recovery,
> that's OK. As explained above, all the callers must tolerate that anyway.
> But if it returns 'false', then it performs a full memory barrier, which
> should ensure that it sees any other shared variables as it is after the
> startup process cleared SharedRecoveryInProgress (notably,
> XLogCtl->ThisTimeLineID).

I'd argue that we should also remove the spinlock in StartupXLOG and
replace it with a write barrier. Obviously not for performance reasons,
but because somebody might add more code to run under that spinlock.

Looks good otherwise, although a read memory barrier ought to suffice.

Greetings,

Andres Freund

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