From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Andres Freund <andres@anarazel.de>
Cc: Andreas Seltenreich <seltenreich@gmx.de>
Cc: pgsql-hackers@postgresql.org, Michael Paquier <michael.paquier@gmail.com>
Subject: Re: [sqlsmith] stuck spinlock in pg_stat_get_wal_receiver after OOM
Date: Tue, 3 Oct 2017 16:07:14 +0200
Message-ID: <20171003140714.ozcduitwplj3ropj@alvherre.pgsql> (raw)
In-Reply-To: <1409.1506981260@sss.pgh.pa.us>
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-hackers>
Fixed the pstrdup problem by replacing with strlcpy() to stack-allocated
variables (rather than palloc + memcpy as proposed in Michael's patch).
About the other problems:
Tom Lane wrote:
> I took a quick look through walreceiver.c, and there are a couple of
> obvious problems of the same ilk in WalReceiverMain, which is doing this:
>
> walrcv->lastMsgSendTime = walrcv->lastMsgReceiptTime = walrcv->latestWalEndTime = GetCurrentTimestamp();
>
> (ie, a potential kernel call) inside a spinlock. But there seems no
> real problem with just collecting the timestamp before we enter that
> critical section.
Done that way.
> I also don't especially like the fact that just above there it reaches
> elog(PANIC) with the lock still held, though at least that's a case that
> should never happen.
Fixed by releasing spinlock just before elog.
> Further down, it's doing a pfree() inside the spinlock, apparently
> for no other reason than to save one "if (tmp_conninfo)".
Fixed.
> I don't especially like the Asserts inside spinlocks, either. Personally,
> I think if those conditions are worth testing then they're worth testing
> for real (in production). Variables that are manipulated by multiple
> processes are way more likely to assume unexpected states than local
> variables.
I didn't change these. It doesn't look to me that these asserts are
worth very much as production code.
> I'm also rather befuddled by the fact that this code sets and clears
> walrcv->latch outside the critical sections. If that field is used
> by any other process, surely that's completely unsafe. If it isn't,
> why is it being kept in shared memory?
I think the latch is only used locally. Seems that it was only put in
shmem to avoid a separate variable ...
--
Álvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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Cc: alvherre@alvh.no-ip.org, tgl@sss.pgh.pa.us, andres@anarazel.de, seltenreich@gmx.de, michael.paquier@gmail.com
Subject: Re: [sqlsmith] stuck spinlock in pg_stat_get_wal_receiver after OOM
In-Reply-To: <20171003140714.ozcduitwplj3ropj@alvherre.pgsql>
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