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lwlock: Fix, currently harmless, bug in LWLockWakeup()

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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
This commit is contained in:
Andres Freund
2025-11-24 17:37:09 -05:00
parent ea36c2f6a9
commit 332693e759
1 changed files with 1 additions and 1 deletions
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2
src/backend/storage/lmgr/lwlock.c
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@@ -1029,7 +1029,7 @@ LWLockWakeup(LWLock *lock)
else else
desired_state &= ~LW_FLAG_RELEASE_OK; 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_HAS_WAITERS;
desired_state &= ~LW_FLAG_LOCKED; /* release lock */ desired_state &= ~LW_FLAG_LOCKED; /* release lock */