From: Tom Lane <tgl@sss.pgh.pa.us>
To: Alvaro Herrera <alvherre@alvh.no-ip.org>
cc: Andres Freund <andres@anarazel.de>,
        Andreas Seltenreich <seltenreich@gmx.de>,
 pgsql-hackers@postgresql.org,
        Michael Paquier <michael.paquier@gmail.com>,
        Peter Eisentraut <peter.eisentraut@2ndquadrant.com>,
        Petr Jelinek <petr@2ndquadrant.com>
Subject: Re: [sqlsmith] stuck spinlock in pg_stat_get_wal_receiver after OOM
In-reply-to: <6526.1507049903@sss.pgh.pa.us>
References: <20171003145023.nc2ffduj3qvy4b5b@alvherre.pgsql>
 <22735.1507048202@sss.pgh.pa.us> <6526.1507049903@sss.pgh.pa.us>
Comments: In-reply-to Tom Lane <tgl@sss.pgh.pa.us>
	message dated "Tue, 03 Oct 2017 12:58:23 -0400"
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Date: Tue, 03 Oct 2017 13:44:12 -0400
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I wrote:
>> So that's trouble waiting to happen, for sure.  At the very least we
>> need to do a single fetch of WalRcv->latch, not two.  I wonder whether
>> even that is sufficient, though: this coding requires an atomic fetch of
>> a pointer, which is something we generally don't assume to be safe.

BTW, I had supposed that this bug was of long standing, but actually it's
new in v10, dating to 597a87ccc9a6fa8af7f3cf280b1e24e41807d555.  Before
that walreceiver start/stop just changed the owner of a long-lived shared
latch, and there was no question of stale pointers.

I considered reverting that decision, but the reason for it seems to have
been to let libpqwalreceiver.c manipulate MyProc->procLatch rather than
having to know about a custom latch.  That's probably a sufficient reason
to justify some squishiness in the wakeup logic.  Still, we might want to
revisit it if we find any other problems here.

			regards, tom lane


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