From 332693e7596909516849e9d7622fbca21b575f28 Mon Sep 17 00:00:00 2001 From: Andres Freund Date: Mon, 24 Nov 2025 17:37:09 -0500 Subject: [PATCH] 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 --- src/backend/storage/lmgr/lwlock.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/src/backend/storage/lmgr/lwlock.c b/src/backend/storage/lmgr/lwlock.c index 28a540961ea..bb2d894393e 100644 --- a/src/backend/storage/lmgr/lwlock.c +++ b/src/backend/storage/lmgr/lwlock.c @@ -1029,7 +1029,7 @@ LWLockWakeup(LWLock *lock) else desired_state &= ~LW_FLAG_RELEASE_OK; - if (proclist_is_empty(&wakeup)) + if (proclist_is_empty(&lock->waiters)) desired_state &= ~LW_FLAG_HAS_WAITERS; desired_state &= ~LW_FLAG_LOCKED; /* release lock */