From: Andres Freund <andres@anarazel.de>
To: pgsql-committers@lists.postgresql.org
Subject: pgsql: lwlock: Fix, currently harmless, bug in LWLockWakeup()
Date: Mon, 24 Nov 2025 23:12:12 +0000
Message-ID: <E1vNfjA-001FH5-0W@gemulon.postgresql.org> (raw)
lwlock: Fix, currently harmless, bug in LWLockWakeup()
Accidentally the code in LWLockWakeup() checked the list of to-be-woken up
processes to see if LW_FLAG_HAS_WAITERS should be unset. That means that
HAS_WAITERS would not get unset immediately, but only during the next,
unnecessary, call to LWLockWakeup().
Luckily, as the code stands, this is just a small efficiency issue.
However, if there were (as in a patch of mine) a case in which LWLockWakeup()
would not find any backend to wake, despite the wait list not being empty,
we'd wrongly unset LW_FLAG_HAS_WAITERS, leading to potentially hanging.
While the consequences in the backbranches are limited, the code as-is
confusing, and it is possible that there are workloads where the additional
wait list lock acquisitions hurt, therefore backpatch.
Discussion: https://postgr.es/m/fvfmkr5kk4nyex56ejgxj3uzi63isfxovp2biecb4bspbjrze7@az2pljabhnff
Backpatch-through: 14
Branch
------
REL_16_STABLE
Details
-------
https://git.postgresql.org/pg/commitdiff/89c8a1b9069f7f1375b114c85258289f77b03a0f
Modified Files
--------------
src/backend/storage/lmgr/lwlock.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
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To: pgsql-committers@postgresql.org
Cc: andres@anarazel.de, pgsql-committers@lists.postgresql.org
Subject: Re: pgsql: lwlock: Fix, currently harmless, bug in LWLockWakeup()
In-Reply-To: <E1vNfjA-001FH5-0W@gemulon.postgresql.org>
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