Split use of SerialSLRULock, creating SerialControlLock

predicate.c has been using SerialSLRULock (the control lock for its SLRU
structure) to coordinate access to SerialControlData, another of its
numerous shared memory structures; this is unnecessary and confuses
further SLRU scalability work.  Create a separate LWLock to cover
SerialControlData.

Extracted from a larger patch from the same author, and some additional
changes by Álvaro.

Author: Dilip Kumar <[email protected]>
Discussion: https://postgr.es/m/CAFiTN-vzDvNz=ExGXz6gdyjtzGixKSqs0mKHMmaQ8sOSEFZ33A@mail.gmail.com

2 files changed, 31 insertions, 11 deletions

diff --git a/src/backend/storage/lmgr/lwlocknames.txt b/src/backend/storage/lmgr/lwlocknames.txt
index a0163b21879..3d59d3646e5 100644
--- a/src/backend/storage/lmgr/lwlocknames.txt
+++ b/src/backend/storage/lmgr/lwlocknames.txt
@@ -57,3 +57,4 @@ WaitEventExtensionLock				48
 WALSummarizerLock					49
 DSMRegistryLock						50
 InjectionPointLock				51
+SerialControlLock					52
diff --git a/src/backend/storage/lmgr/predicate.c b/src/backend/storage/lmgr/predicate.c
index ee5ea1175c7..eed63a05ed8 100644
--- a/src/backend/storage/lmgr/predicate.c
+++ b/src/backend/storage/lmgr/predicate.c
@@ -134,6 +134,11 @@
  *	SerializableXactHashLock
  *		- Protects both PredXact and SerializableXidHash.
  *
+ *	SerialControlLock
+ *		- Protects SerialControlData members
+ *
+ *	SerialSLRULock
+ *		- Protects SerialSlruCtl
  *
  * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
@@ -828,9 +833,11 @@ SerialInit(void)
 		/*
 		 * Set control information to reflect empty SLRU.
 		 */
+		LWLockAcquire(SerialControlLock, LW_EXCLUSIVE);
 		serialControl->headPage = -1;
 		serialControl->headXid = InvalidTransactionId;
 		serialControl->tailXid = InvalidTransactionId;
+		LWLockRelease(SerialControlLock);
 	}
 }
 
@@ -852,7 +859,12 @@ SerialAdd(TransactionId xid, SerCommitSeqNo minConflictCommitSeqNo)
 
 	targetPage = SerialPage(xid);
 
-	LWLockAcquire(SerialSLRULock, LW_EXCLUSIVE);
+	/*
+	 * In this routine, we must hold both SerialControlLock and SerialSLRULock
+	 * simultaneously while making the SLRU data catch up with the new state
+	 * that we determine.
+	 */
+	LWLockAcquire(SerialControlLock, LW_EXCLUSIVE);
 
 	/*
 	 * If no serializable transactions are active, there shouldn't be anything
@@ -886,6 +898,8 @@ SerialAdd(TransactionId xid, SerCommitSeqNo minConflictCommitSeqNo)
 	if (isNewPage)
 		serialControl->headPage = targetPage;
 
+	LWLockAcquire(SerialSLRULock, LW_EXCLUSIVE);
+
 	if (isNewPage)
 	{
 		/* Initialize intervening pages. */
@@ -903,6 +917,7 @@ SerialAdd(TransactionId xid, SerCommitSeqNo minConflictCommitSeqNo)
 	SerialSlruCtl->shared->page_dirty[slotno] = true;
 
 	LWLockRelease(SerialSLRULock);
+	LWLockRelease(SerialControlLock);
 }
 
 /*
@@ -920,10 +935,10 @@ SerialGetMinConflictCommitSeqNo(TransactionId xid)
 
 	Assert(TransactionIdIsValid(xid));
 
-	LWLockAcquire(SerialSLRULock, LW_SHARED);
+	LWLockAcquire(SerialControlLock, LW_SHARED);
 	headXid = serialControl->headXid;
 	tailXid = serialControl->tailXid;
-	LWLockRelease(SerialSLRULock);
+	LWLockRelease(SerialControlLock);
 
 	if (!TransactionIdIsValid(headXid))
 		return 0;
@@ -954,7 +969,7 @@ SerialGetMinConflictCommitSeqNo(TransactionId xid)
 static void
 SerialSetActiveSerXmin(TransactionId xid)
 {
-	LWLockAcquire(SerialSLRULock, LW_EXCLUSIVE);
+	LWLockAcquire(SerialControlLock, LW_EXCLUSIVE);
 
 	/*
 	 * When no sxacts are active, nothing overlaps, set the xid values to
@@ -966,7 +981,7 @@ SerialSetActiveSerXmin(TransactionId xid)
 	{
 		serialControl->tailXid = InvalidTransactionId;
 		serialControl->headXid = InvalidTransactionId;
-		LWLockRelease(SerialSLRULock);
+		LWLockRelease(SerialControlLock);
 		return;
 	}
 
@@ -984,7 +999,7 @@ SerialSetActiveSerXmin(TransactionId xid)
 		{
 			serialControl->tailXid = xid;
 		}
-		LWLockRelease(SerialSLRULock);
+		LWLockRelease(SerialControlLock);
 		return;
 	}
 
@@ -993,7 +1008,7 @@ SerialSetActiveSerXmin(TransactionId xid)
 
 	serialControl->tailXid = xid;
 
-	LWLockRelease(SerialSLRULock);
+	LWLockRelease(SerialControlLock);
 }
 
 /*
@@ -1007,12 +1022,12 @@ CheckPointPredicate(void)
 {
 	int			truncateCutoffPage;
 
-	LWLockAcquire(SerialSLRULock, LW_EXCLUSIVE);
+	LWLockAcquire(SerialControlLock, LW_EXCLUSIVE);
 
 	/* Exit quickly if the SLRU is currently not in use. */
 	if (serialControl->headPage < 0)
 	{
-		LWLockRelease(SerialSLRULock);
+		LWLockRelease(SerialControlLock);
 		return;
 	}
 
@@ -1072,9 +1087,13 @@ CheckPointPredicate(void)
 		serialControl->headPage = -1;
 	}
 
-	LWLockRelease(SerialSLRULock);
+	LWLockRelease(SerialControlLock);
 
-	/* Truncate away pages that are no longer required */
+	/*
+	 * Truncate away pages that are no longer required.  Note that no
+	 * additional locking is required, because this is only called as part of
+	 * a checkpoint, and the validity limits have already been determined.
+	 */
 	SimpleLruTruncate(SerialSlruCtl, truncateCutoffPage);
 
 	/*