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From: Nathan Bossart <nathandbossart@gmail.com>
To: pgsql-hackers@postgresql.org
Subject: make LWLockCounter a global variable
Date: Thu, 28 Aug 2025 16:35:25 -0500
Message-ID: <aLDLnan9gNCS9fHx@nathan> (raw)

In lwlock.c, uses of LWLockCounter must first calculate its address in
shared memory with something like this:

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

This appears to have been started by commit 82e861f in order to fix
EXEC_BACKEND builds, but it could also be fixed by adding it to the
BackendParameters struct.  I find the current approach somewhat difficult
to read and understand, so I'd like to switch to the latter approach.  This
is admittedly just nitpicking...

-- 
nathan

Attachments:

v1-0001-Make-LWLockCounter-a-global-variable.patch (text/plain, 3.7K) download
From d4b1847f7e7d1ef4b32b4a5304b2a55cabe55f56 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Thu, 28 Aug 2025 16:22:02 -0500
Subject: [PATCH v1 1/1] Make LWLockCounter a global variable.

---
 src/backend/postmaster/launch_backend.c |  3 +++
 src/backend/storage/lmgr/lwlock.c       | 15 ++++-----------
 src/include/storage/lwlock.h            |  1 +
 3 files changed, 8 insertions(+), 11 deletions(-)

diff --git a/src/backend/postmaster/launch_backend.c b/src/backend/postmaster/launch_backend.c
index bf6b55ee830..cd9547b03a3 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 c80b43f1f55..09401515d3a 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);
@@ -423,13 +424,14 @@ 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 */
@@ -437,13 +439,6 @@ CreateLWLocks(void)
 
 		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 +569,7 @@ 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 5e717765764..f9cf57f8d26 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 (Apple Git-154)

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