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

diff --git a/src/backend/storage/lmgr/lwlock.c b/src/backend/storage/lmgr/lwlock.c
index b017880f5e4510d168ea1222eb208d74222e126c..255cfa8fa95e3b5abb55c08ceef958b55720b0c9 100644 (file)
--- a/src/backend/storage/lmgr/lwlock.c
+++ b/src/backend/storage/lmgr/lwlock.c
@@ -998,7 +998,7 @@ 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 */