Re: pgsql: Improve performance of subsystems on top of SLRU

Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)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