From: | Andres Freund <andres(at)2ndquadrant(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: Latches and barriers |
Date: | 2015-01-12 16:27:30 |
Message-ID: | 20150112162730.GC2092@awork2.anarazel.de |
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Lists: | pgsql-hackers |
On 2015-01-12 11:03:42 -0500, Tom Lane wrote:
> Andres Freund <andres(at)2ndquadrant(dot)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 & Services
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