| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> |
| Cc: | Andres Freund <andres(at)anarazel(dot)de>, Andreas Seltenreich <seltenreich(at)gmx(dot)de>, pgsql-hackers(at)postgresql(dot)org, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Peter Eisentraut <peter(dot)eisentraut(at)2ndquadrant(dot)com>, Petr Jelinek <petr(at)2ndquadrant(dot)com> |
| Subject: | Re: [sqlsmith] stuck spinlock in pg_stat_get_wal_receiver after OOM |
| Date: | 2017-10-03 17:44:12 |
| Message-ID: | 23454.1507052652@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
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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