From 67fcf48c3bb3cc3cf51788332701ffb694fb108b Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Fri, 29 Aug 2025 12:13:37 -0500
Subject: [PATCH] Make LWLockCounter a global variable.

Using the LWLockCounter requires first calculating its address in
shared memory like this:

	LWLockCounter = (int *) ((char *) MainLWLockArray - sizeof(int));

Commit 82e861fbe1 started this trend in order to fix EXEC_BACKEND
builds, but it could also be fixed by adding it to the
BackendParameters struct.  The current approach is somewhat
difficult to follow, so this commit switches to the latter.  While
at it, swap around the code in LWLockShmemSize() to match the order
of assignments in CreateLWLocks() for added readability.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/aLDLnan9gNCS9fHx%40nathan
---
 src/backend/postmaster/launch_backend.c |  3 +++
 src/backend/storage/lmgr/lwlock.c       | 24 ++++++++----------------
 src/include/storage/lwlock.h            |  1 +
 3 files changed, 12 insertions(+), 16 deletions(-)

diff --git a/src/backend/postmaster/launch_backend.c b/src/backend/postmaster/launch_backend.c
index bf6b55ee83..cd9547b03a 100644
--- a/src/backend/postmaster/launch_backend.c
+++ b/src/backend/postmaster/launch_backend.c
@@ -102,6 +102,7 @@ typedef struct
 #endif
 	int			NamedLWLockTrancheRequests;
 	NamedLWLockTranche *NamedLWLockTrancheArray;
+	int		   *LWLockCounter;
 	LWLockPadded *MainLWLockArray;
 	slock_t    *ProcStructLock;
 	PROC_HDR   *ProcGlobal;
@@ -761,6 +762,7 @@ save_backend_variables(BackendParameters *param,
 
 	param->NamedLWLockTrancheRequests = NamedLWLockTrancheRequests;
 	param->NamedLWLockTrancheArray = NamedLWLockTrancheArray;
+	param->LWLockCounter = LWLockCounter;
 	param->MainLWLockArray = MainLWLockArray;
 	param->ProcStructLock = ProcStructLock;
 	param->ProcGlobal = ProcGlobal;
@@ -1021,6 +1023,7 @@ restore_backend_variables(BackendParameters *param)
 
 	NamedLWLockTrancheRequests = param->NamedLWLockTrancheRequests;
 	NamedLWLockTrancheArray = param->NamedLWLockTrancheArray;
+	LWLockCounter = param->LWLockCounter;
 	MainLWLockArray = param->MainLWLockArray;
 	ProcStructLock = param->ProcStructLock;
 	ProcGlobal = param->ProcGlobal;
diff --git a/src/backend/storage/lmgr/lwlock.c b/src/backend/storage/lmgr/lwlock.c
index c80b43f1f5..a4aecd1fbc 100644
--- a/src/backend/storage/lmgr/lwlock.c
+++ b/src/backend/storage/lmgr/lwlock.c
@@ -196,6 +196,7 @@ int			NamedLWLockTrancheRequests = 0;
 
 /* points to data in shared memory: */
 NamedLWLockTranche *NamedLWLockTrancheArray = NULL;
+int		   *LWLockCounter = NULL;
 
 static void InitializeLWLocks(void);
 static inline void LWLockReportWaitStart(LWLock *lock);
@@ -397,11 +398,11 @@ LWLockShmemSize(void)
 	/* Calculate total number of locks needed in the main array. */
 	numLocks += NumLWLocksForNamedTranches();
 
-	/* Space for the LWLock array. */
-	size = mul_size(numLocks, sizeof(LWLockPadded));
-
 	/* Space for dynamic allocation counter, plus room for alignment. */
-	size = add_size(size, sizeof(int) + LWLOCK_PADDED_SIZE);
+	size = sizeof(int) + LWLOCK_PADDED_SIZE;
+
+	/* Space for the LWLock array. */
+	size = add_size(size, mul_size(numLocks, sizeof(LWLockPadded)));
 
 	/* space for named tranches. */
 	size = add_size(size, mul_size(NamedLWLockTrancheRequests, sizeof(NamedLWLockTranche)));
@@ -423,27 +424,20 @@ CreateLWLocks(void)
 	if (!IsUnderPostmaster)
 	{
 		Size		spaceLocks = LWLockShmemSize();
-		int		   *LWLockCounter;
 		char	   *ptr;
 
 		/* Allocate space */
 		ptr = (char *) ShmemAlloc(spaceLocks);
 
-		/* Leave room for dynamic allocation of tranches */
+		/* Initialize the dynamic-allocation counter for tranches */
+		LWLockCounter = (int *) ptr;
+		*LWLockCounter = LWTRANCHE_FIRST_USER_DEFINED;
 		ptr += sizeof(int);
 
 		/* Ensure desired alignment of LWLock array */
 		ptr += LWLOCK_PADDED_SIZE - ((uintptr_t) ptr) % LWLOCK_PADDED_SIZE;
-
 		MainLWLockArray = (LWLockPadded *) ptr;
 
-		/*
-		 * Initialize the dynamic-allocation counter for tranches, which is
-		 * stored just before the first LWLock.
-		 */
-		LWLockCounter = (int *) ((char *) MainLWLockArray - sizeof(int));
-		*LWLockCounter = LWTRANCHE_FIRST_USER_DEFINED;
-
 		/* Initialize all LWLocks */
 		InitializeLWLocks();
 	}
@@ -574,9 +568,7 @@ int
 LWLockNewTrancheId(void)
 {
 	int			result;
-	int		   *LWLockCounter;
 
-	LWLockCounter = (int *) ((char *) MainLWLockArray - sizeof(int));
 	/* We use the ShmemLock spinlock to protect LWLockCounter */
 	SpinLockAcquire(ShmemLock);
 	result = (*LWLockCounter)++;
diff --git a/src/include/storage/lwlock.h b/src/include/storage/lwlock.h
index 5e71776576..f9cf57f8d2 100644
--- a/src/include/storage/lwlock.h
+++ b/src/include/storage/lwlock.h
@@ -82,6 +82,7 @@ typedef struct NamedLWLockTranche
 
 extern PGDLLIMPORT NamedLWLockTranche *NamedLWLockTrancheArray;
 extern PGDLLIMPORT int NamedLWLockTrancheRequests;
+extern PGDLLIMPORT int *LWLockCounter;
 
 /*
  * It's a bit odd to declare NUM_BUFFER_PARTITIONS and NUM_LOCK_PARTITIONS
-- 
2.39.5

