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pgsql: Avoid locking when an LSN waiter is already removed 2+ messages / 1 participants [nested] [flat]
* pgsql: Avoid locking when an LSN waiter is already removed @ 2026-08-18 12:02 Alexander Korotkov <akorotkov@postgresql.org> 0 siblings, 0 replies; 2+ messages in thread From: Alexander Korotkov @ 2026-08-18 12:02 UTC (permalink / raw) To: pgsql-committers@lists.postgresql.org 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(-) ^ permalink raw reply [nested|flat] 2+ messages in thread
* pgsql: Avoid locking when an LSN waiter is already removed @ 2026-08-18 12:02 Alexander Korotkov <akorotkov@postgresql.org> 0 siblings, 0 replies; 2+ messages in thread From: Alexander Korotkov @ 2026-08-18 12:02 UTC (permalink / raw) To: pgsql-committers@lists.postgresql.org 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 ------ REL_19_STABLE Details ------- https://git.postgresql.org/pg/commitdiff/58c48b4ec42583c69b3a7e9c6cf7649b38fc8151 Modified Files -------------- src/backend/access/transam/xlogwait.c | 27 +++++++++++++++++---------- 1 file changed, 17 insertions(+), 10 deletions(-) ^ permalink raw reply [nested|flat] 2+ messages in thread
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