From: Tom Lane <tgl@sss.pgh.pa.us>
To: Alvaro Herrera <alvherre@alvh.no-ip.org>
cc: pgsql-hackers@lists.postgresql.org, Dilip Kumar <dilipbalaut@gmail.com>
Subject: Re: pgsql: Improve performance of subsystems on top of SLRU
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Comments: In-reply-to Alvaro Herrera <alvherre@alvh.no-ip.org>
	message dated "Sun, 03 Mar 2024 15:29:22 +0100"
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Date: Sun, 03 Mar 2024 16:14:49 -0500
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Alvaro Herrera <alvherre@alvh.no-ip.org> writes:
> And I think it's correct that this is somewhat bogus, or at least
> confusing: the only way to have control back here on line 1371 after
> having executed once is via the "goto retry" line below; and there we
> release "prevlock" and set it to NULL beforehand, so it's impossible for
> prevlock to be NULL.  Looking closer I think this code is all confused,
> so I suggest to rework it as shown in the attached patch.

This is certainly simpler, but I notice that it holds the current
LWLock across the line

 	ptr = (MultiXactMember *) palloc(length * sizeof(MultiXactMember));

where the old code did not.  Could the palloc take long enough that
holding the lock is bad?

Also, with this coding the "lock = NULL;" assignment just before
"goto retry" is a dead store.  Not sure if Coverity or other static
analyzers would whine about that.

			regards, tom lane