Re: Latches and barriers

On 2015-01-12 11:03:42 -0500, Tom Lane wrote:
> Andres Freund <andres@2ndquadrant.com> writes:
> > While it might not be required for existing latch uses (I'm *not* sure
> > that's true)

I think at least syncrep.c might not be correct. In SyncRepWakeQueue()
it sets PGPROC->syncRepState without the necessary barriers (via locks),
although it does use them in SyncRepWaitForLSN().

It is, perhaps surprisingly to many, not sufficient to take a spinlock,
change the flag, release it and then set the latch - the release alone
doesn't guarantee a sufficient barrier unless looking at the flag is
also protected by the spinlock.

> I still think that we should fix those XXX by actually
> > using barriers now that we have them. I don't think we want every
> > callsite worry about using barriers.
> 
> > Agreed?
> 
> Yeah, now that we have barrier code we think works, we should definitely
> put some in there.  The only reason it's like that is we didn't have
> any real barrier support at the time.

Master only though? If we decide we need it earlier, we can backport
that commit lateron...

Greetings,

Andres Freund

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