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 <dilip.kumar@enterprisedb.com>
Discussion: https://postgr.es/m/CAFiTN-vzDvNz=ExGXz6gdyjtzGixKSqs0mKHMmaQ8sOSEFZ33A@mail.gmail.com

src/backend/storage/lmgr/lwlocknames.txt

@@ -57,3 +57,4 @@ WaitEventExtensionLock				48
 WALSummarizerLock					49
 DSMRegistryLock						50
 InjectionPointLock				51
+SerialControlLock					52

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);
 
 	/*

src/backend/utils/activity/wait_event_names.txt

@@ -331,6 +331,7 @@ WaitEventExtension	"Waiting to read or update custom wait events information for
 WALSummarizer	"Waiting to read or update WAL summarization state."
 DSMRegistry	"Waiting to read or update the dynamic shared memory registry."
 InjectionPoint	"Waiting to read or update information related to injection points."
+SerialControl	"Waiting to read or update shared <filename>pg_serial</filename> state."
 
 #
 # END OF PREDEFINED LWLOCKS (DO NOT CHANGE THIS LINE)