From 57306a6a2a4e7300206d3a5e40a1787b427765b8 Mon Sep 17 00:00:00 2001
From: Andres Freund <andres@anarazel.de>
Date: Thu, 27 Oct 2022 10:18:13 -0700
Subject: [PATCH v1 01/12] wip: lwlock: fix quadratic behaviour with very long
 wait lists

Author:
Reviewed-by:
Discussion: https://postgr.es/m/
Backpatch:
---
 src/include/storage/proc.h        |  3 ++-
 src/backend/storage/lmgr/lwlock.c | 29 +++++++++++++++--------------
 2 files changed, 17 insertions(+), 15 deletions(-)

diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 8d096fdeeb1..9a2615666a1 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -217,7 +217,8 @@ struct PGPROC
 	bool		recoveryConflictPending;
 
 	/* Info about LWLock the process is currently waiting for, if any. */
-	bool		lwWaiting;		/* true if waiting for an LW lock */
+	int			lwWaiting;		/* 0 if not waiting, 1 if on waitlist, 2 if
+								 * waiting to be woken */
 	uint8		lwWaitMode;		/* lwlock mode being waited for */
 	proclist_node lwWaitLink;	/* position in LW lock wait list */
 
diff --git a/src/backend/storage/lmgr/lwlock.c b/src/backend/storage/lmgr/lwlock.c
index d274c9b1dc9..2d1c8fae06e 100644
--- a/src/backend/storage/lmgr/lwlock.c
+++ b/src/backend/storage/lmgr/lwlock.c
@@ -987,6 +987,9 @@ LWLockWakeup(LWLock *lock)
 			wokeup_somebody = true;
 		}
 
+		/* signal that the process isn't on the wait list anymore */
+		waiter->lwWaiting = 2;
+
 		/*
 		 * Once we've woken up an exclusive lock, there's no point in waking
 		 * up anybody else.
@@ -1044,7 +1047,7 @@ LWLockWakeup(LWLock *lock)
 		 * another lock.
 		 */
 		pg_write_barrier();
-		waiter->lwWaiting = false;
+		waiter->lwWaiting = 0;
 		PGSemaphoreUnlock(waiter->sem);
 	}
 }
@@ -1073,7 +1076,7 @@ LWLockQueueSelf(LWLock *lock, LWLockMode mode)
 	/* setting the flag is protected by the spinlock */
 	pg_atomic_fetch_or_u32(&lock->state, LW_FLAG_HAS_WAITERS);
 
-	MyProc->lwWaiting = true;
+	MyProc->lwWaiting = 1;
 	MyProc->lwWaitMode = mode;
 
 	/* LW_WAIT_UNTIL_FREE waiters are always at the front of the queue */
@@ -1101,7 +1104,6 @@ static void
 LWLockDequeueSelf(LWLock *lock)
 {
 	bool		found = false;
-	proclist_mutable_iter iter;
 
 #ifdef LWLOCK_STATS
 	lwlock_stats *lwstats;
@@ -1114,17 +1116,14 @@ LWLockDequeueSelf(LWLock *lock)
 	LWLockWaitListLock(lock);
 
 	/*
-	 * Can't just remove ourselves from the list, but we need to iterate over
-	 * all entries as somebody else could have dequeued us.
+	 * Remove ourselves from the waitlist, unless we've already been
+	 * removed. The removal happens with the wait list lock held, so there's
+	 * no race in this check.
 	 */
-	proclist_foreach_modify(iter, &lock->waiters, lwWaitLink)
+	if (MyProc->lwWaiting == 1)
 	{
-		if (iter.cur == MyProc->pgprocno)
-		{
-			found = true;
-			proclist_delete(&lock->waiters, iter.cur, lwWaitLink);
-			break;
-		}
+		proclist_delete(&lock->waiters, MyProc->pgprocno, lwWaitLink);
+		found = true;
 	}
 
 	if (proclist_is_empty(&lock->waiters) &&
@@ -1138,7 +1137,7 @@ LWLockDequeueSelf(LWLock *lock)
 
 	/* clear waiting state again, nice for debugging */
 	if (found)
-		MyProc->lwWaiting = false;
+		MyProc->lwWaiting = 0;
 	else
 	{
 		int			extraWaits = 0;
@@ -1772,6 +1771,8 @@ LWLockUpdateVar(LWLock *lock, uint64 *valptr, uint64 val)
 
 		proclist_delete(&lock->waiters, iter.cur, lwWaitLink);
 		proclist_push_tail(&wakeup, iter.cur, lwWaitLink);
+
+		waiter->lwWaiting = 2;
 	}
 
 	/* We are done updating shared state of the lock itself. */
@@ -1787,7 +1788,7 @@ LWLockUpdateVar(LWLock *lock, uint64 *valptr, uint64 val)
 		proclist_delete(&wakeup, iter.cur, lwWaitLink);
 		/* check comment in LWLockWakeup() about this barrier */
 		pg_write_barrier();
-		waiter->lwWaiting = false;
+		waiter->lwWaiting = 0;
 		PGSemaphoreUnlock(waiter->sem);
 	}
 }
-- 
2.38.0