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

anarazel · anarazel · commit 89c8a1b9069f · 2025-11-24T17:39:58.000-05:00
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
diff --git a/src/backend/storage/lmgr/lwlock.c b/src/backend/storage/lmgr/lwlock.c
@@ -1019,7 +1019,7 @@ 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 */
