From: Alexander Korotkov <akorotkov@postgresql.org>
To: pgsql-committers@lists.postgresql.org
Subject: pgsql: Avoid locking when an LSN waiter is already removed
Date: Tue, 18 Aug 2026 12:02:13 +0000
Message-ID: <E1wwIWC-000000013YP-2AIz@gemulon.postgresql.org> (raw)
Avoid locking when an LSN waiter is already removed
WaitLSNWakeup() removes each selected waiter from its heap and clears
its inHeap flag before setting its latch. When such a waiter later
calls deleteLSNWaiter(), it acquires WaitLSNLock exclusively only to
discover that there is nothing left to remove. Waking many waiters can
therefore make them serialize on the lock for no useful work.
Check inHeap before acquiring WaitLSNLock. A lockless false value is
conclusive because only the owning backend can change inHeap from false
to true. A concurrent waker can only clear it. A stale true value
falls through to the existing recheck under the lock.
WaitLSNCleanup() performed the same lockless check before calling
deleteLSNWaiter(). Drop it there, as it is now redundant.
Author: Xuneng Zhou <xunengzhou@gmail.com>
Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com>
Discussion: https://postgr.es/m/CABPTF7UtW_cAa%3DQh4RDfKiUqu3pJJE22ai9tbWJVERbeRyssLw%40mail.gmail.com
Backpatch-through: 19
Branch
------
master
Details
-------
https://git.postgresql.org/pg/commitdiff/0c8dd518a58fb7b65c992d3bb6ce21aa3f4fa9fb
Modified Files
--------------
src/backend/access/transam/xlogwait.c | 27 +++++++++++++++++----------
1 file changed, 17 insertions(+), 10 deletions(-)
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To: pgsql-committers@postgresql.org
Cc: akorotkov@postgresql.org, pgsql-committers@lists.postgresql.org
Subject: Re: pgsql: Avoid locking when an LSN waiter is already removed
In-Reply-To: <E1wwIWC-000000013YP-2AIz@gemulon.postgresql.org>
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