Subject: [PATCH v9 04/10] lwlock: Invert meaning of LW_FLAG_RELEASE_OK

From: Andres Freund <[email-masked]>
Date: Mon, 5 Jan 2026 20:40:38 -0500

Instead of setting a flag whenever a lock release is supposed to wake up
waiters - the majority of the time - set a flag whenever wakeups are
inhibited. The motivation for this that in an upcoming commit, buffer content
locks are implemented separately from lwlocks, and for buffer content locks it
is useful to be able to reset all buffer flags when a buffer invalidated,
alternatively we would have to set the release-ok flag when making a buffer
valid.  It seems good to keep the implementation of lwlocks and buffer content
locks as similar as reasonably possible.

Author:
Reviewed-by:
Discussion: https://postgr.es/m/
Backpatch:
---
 src/backend/storage/lmgr/lwlock.c | 42 +++++++++++++++----------------
 1 file changed, 20 insertions(+), 22 deletions(-)

diff --git a/src/backend/storage/lmgr/lwlock.c b/src/backend/storage/lmgr/lwlock.c
index 6a9f86d5025..148309cc186 100644
--- a/src/backend/storage/lmgr/lwlock.c
+++ b/src/backend/storage/lmgr/lwlock.c
@@ -92,7 +92,7 @@

 #define LW_FLAG_HAS_WAITERS			((uint32) 1 << 31)
-#define LW_FLAG_RELEASE_OK			((uint32) 1 << 30)
+#define LW_FLAG_WAKE_IN_PROGRESS	((uint32) 1 << 30)
 #define LW_FLAG_LOCKED				((uint32) 1 << 29)
 #define LW_FLAG_BITS				3
 #define LW_FLAG_MASK				(((1<<LW_FLAG_BITS)-1)<<(32-LW_FLAG_BITS))
@@ -246,14 +246,14 @@ PRINT_LWDEBUG(const char *where, LWLock *lock, LWLockMode mode)
 		ereport(LOG,
 				(errhidestmt(true),
 				 errhidecontext(true),
-				 errmsg_internal("%d: %s(%s %p): excl %u shared %u haswaiters %u waiters %u rOK %d",
+				 errmsg_internal("%d: %s(%s %p): excl %u shared %u haswaiters %u waiters %u waking %d",
 								 MyProcPid,
 								 where, T_NAME(lock), lock,
 								 (state & LW_VAL_EXCLUSIVE) != 0,
 								 state & LW_SHARED_MASK,
 								 (state & LW_FLAG_HAS_WAITERS) != 0,
 								 pg_atomic_read_u32(&lock->nwaiters),
-								 (state & LW_FLAG_RELEASE_OK) != 0)));
+								 (state & LW_FLAG_WAKE_IN_PROGRESS) != 0)));
 	}
 }
 
@@ -700,7 +700,7 @@ LWLockInitialize(LWLock *lock, int tranche_id)
 	/* verify the tranche_id is valid */
 	(void) GetLWTrancheName(tranche_id);
 
-	pg_atomic_init_u32(&lock->state, LW_FLAG_RELEASE_OK);
+	pg_atomic_init_u32(&lock->state, 0);
 #ifdef LOCK_DEBUG
 	pg_atomic_init_u32(&lock->nwaiters, 0);
 #endif
@@ -929,15 +929,13 @@ LWLockWaitListUnlock(LWLock *lock)
 static void
 LWLockWakeup(LWLock *lock)
 {
-	bool		new_release_ok;
+	bool		new_release_in_progress = false;
 	bool		wokeup_somebody = false;
 	proclist_head wakeup;
 	proclist_mutable_iter iter;
 
 	proclist_init(&wakeup);
 
-	new_release_ok = true;
-
 	/* lock wait list while collecting backends to wake up */
 	LWLockWaitListLock(lock);
 
@@ -958,7 +956,7 @@ LWLockWakeup(LWLock *lock)
 			 * that are just waiting for the lock to become free don't retry
 			 * automatically.
 			 */
-			new_release_ok = false;
+			new_release_in_progress = true;
 
 			/*
 			 * Don't wakeup (further) exclusive locks.
@@ -997,10 +995,10 @@ LWLockWakeup(LWLock *lock)
 
 			/* compute desired flags */
 
-			if (new_release_ok)
-				desired_state |= LW_FLAG_RELEASE_OK;
+			if (new_release_in_progress)
+				desired_state |= LW_FLAG_WAKE_IN_PROGRESS;
 			else
-				desired_state &= ~LW_FLAG_RELEASE_OK;
+				desired_state &= ~LW_FLAG_WAKE_IN_PROGRESS;
 
 			if (proclist_is_empty(&lock->waiters))
 				desired_state &= ~LW_FLAG_HAS_WAITERS;
@@ -1131,10 +1129,10 @@ LWLockDequeueSelf(LWLock *lock)
 		 */
 
 		/*
-		 * Reset RELEASE_OK flag if somebody woke us before we removed
-		 * ourselves - they'll have set it to false.
+		 * Clear LW_FLAG_WAKE_IN_PROGRESS if somebody woke us before we
+		 * removed ourselves - they'll have set it.
 		 */
-		pg_atomic_fetch_or_u32(&lock->state, LW_FLAG_RELEASE_OK);
+		pg_atomic_fetch_and_u32(&lock->state, ~LW_FLAG_WAKE_IN_PROGRESS);
 
 		/*
 		 * Now wait for the scheduled wakeup, otherwise our ->lwWaiting would
@@ -1301,7 +1299,7 @@ LWLockAcquire(LWLock *lock, LWLockMode mode)
 		}
 
 		/* Retrying, allow LWLockRelease to release waiters again. */
-		pg_atomic_fetch_or_u32(&lock->state, LW_FLAG_RELEASE_OK);
+		pg_atomic_fetch_and_u32(&lock->state, ~LW_FLAG_WAKE_IN_PROGRESS);
 
 #ifdef LOCK_DEBUG
 		{
@@ -1636,10 +1634,10 @@ LWLockWaitForVar(LWLock *lock, pg_atomic_uint64 *valptr, uint64 oldval,
 		LWLockQueueSelf(lock, LW_WAIT_UNTIL_FREE);
 
 		/*
-		 * Set RELEASE_OK flag, to make sure we get woken up as soon as the
-		 * lock is released.
+		 * Clear LW_FLAG_WAKE_IN_PROGRESS flag, to make sure we get woken up
+		 * as soon as the lock is released.
 		 */
-		pg_atomic_fetch_or_u32(&lock->state, LW_FLAG_RELEASE_OK);
+		pg_atomic_fetch_and_u32(&lock->state, ~LW_FLAG_WAKE_IN_PROGRESS);
 
 		/*
 		 * We're now guaranteed to be woken up if necessary. Recheck the lock
@@ -1852,11 +1850,11 @@ LWLockReleaseInternal(LWLock *lock, LWLockMode mode)
 		TRACE_POSTGRESQL_LWLOCK_RELEASE(T_NAME(lock));
 
 	/*
-	 * We're still waiting for backends to get scheduled, don't wake them up
-	 * again.
+	 * Check if we're still waiting for backends to get scheduled, if so,
+	 * don't wake them up again.
 	 */
-	if ((oldstate & (LW_FLAG_HAS_WAITERS | LW_FLAG_RELEASE_OK)) ==
-		(LW_FLAG_HAS_WAITERS | LW_FLAG_RELEASE_OK) &&
+	if ((oldstate & LW_FLAG_HAS_WAITERS) &&
+		!(oldstate & LW_FLAG_WAKE_IN_PROGRESS) &&
 		(oldstate & LW_LOCK_MASK) == 0)
 		check_waiters = true;
 	else
-- 
2.48.1.76.g4e746b1a31.dirty

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	filename="v9-0005-bufmgr-Make-definitions-related-to-buffer-descrip.patch"