From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: Thu, 26 Mar 2026 21:47:37 +0000 (+0200)
Subject: Use ShmemInitStruct to allocate lwlock.c's shared memory
X-Git-Url: http://git.ipfire.org/gitweb/?a=commitdiff_plain;h=30d432502bf42343ec0f9ad599625cde0cb38e57;p=thirdparty%2Fpostgresql.git

Use ShmemInitStruct to allocate lwlock.c's shared memory

It's nice to have them show up in pg_shmem_allocations like all other
shmem areas. ShmemInitStruct() depends on ShmemIndexLock, but only
after postmaster startup.

Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://www.postgresql.org/message-id/47aaf57e-1b7b-4e12-bda2-0316081ff50e@iki.fi
---

diff --git a/src/backend/postmaster/launch_backend.c b/src/backend/postmaster/launch_backend.c
index 8c134eaca88..434e0643022 100644
--- a/src/backend/postmaster/launch_backend.c
+++ b/src/backend/postmaster/launch_backend.c
@@ -99,8 +99,6 @@ typedef struct
 #ifdef USE_INJECTION_POINTS
 	struct InjectionPointsCtl *ActiveInjectionPoints;
 #endif
-	LWLockTrancheShmemData *LWLockTranches;
-	LWLockPadded *MainLWLockArray;
 	PROC_HDR   *ProcGlobal;
 	PGPROC	   *AuxiliaryProcs;
 	PGPROC	   *PreparedXactProcs;
@@ -726,8 +724,6 @@ save_backend_variables(BackendParameters *param,
 	param->ActiveInjectionPoints = ActiveInjectionPoints;
 #endif
 
-	param->LWLockTranches = LWLockTranches;
-	param->MainLWLockArray = MainLWLockArray;
 	param->ProcGlobal = ProcGlobal;
 	param->AuxiliaryProcs = AuxiliaryProcs;
 	param->PreparedXactProcs = PreparedXactProcs;
@@ -982,8 +978,6 @@ restore_backend_variables(BackendParameters *param)
 	ActiveInjectionPoints = param->ActiveInjectionPoints;
 #endif
 
-	LWLockTranches = param->LWLockTranches;
-	MainLWLockArray = param->MainLWLockArray;
 	ProcGlobal = param->ProcGlobal;
 	AuxiliaryProcs = param->AuxiliaryProcs;
 	PreparedXactProcs = param->PreparedXactProcs;
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 3d3f153809b..d692d419846 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -250,9 +250,9 @@ static void
 CreateOrAttachShmemStructs(void)
 {
 	/*
-	 * Now initialize LWLocks, which do shared memory allocation.
+	 * Set up LWLocks.  They are needed by most other subsystems.
 	 */
-	CreateLWLocks();
+	LWLockShmemInit();
 
 	dsm_shmem_init();
 	DSMRegistryShmemInit();
diff --git a/src/backend/storage/lmgr/lwlock.c b/src/backend/storage/lmgr/lwlock.c
index f9e368770b7..241f1f08430 100644
--- a/src/backend/storage/lmgr/lwlock.c
+++ b/src/backend/storage/lmgr/lwlock.c
@@ -193,9 +193,12 @@ typedef struct LWLockTrancheShmemData
 	int			num_user_defined;	/* 'user_defined' entries in use */
 
 	slock_t		lock;			/* protects the above */
+
+	/* Size of MainLWLockArray */
+	int			num_main_array_locks;
 } LWLockTrancheShmemData;
 
-LWLockTrancheShmemData *LWLockTranches;
+static LWLockTrancheShmemData *LWLockTranches;
 
 /* backend-local copy of NamedLWLockTranches->num_user_defined */
 static int	LocalNumUserDefinedTranches;
@@ -426,23 +429,32 @@ LWLockShmemSize(void)
  * and initialize it.
  */
 void
-CreateLWLocks(void)
+LWLockShmemInit(void)
 {
 	int			numLocks;
+	bool		found;
 
-	if (!IsUnderPostmaster)
+	LWLockTranches = (LWLockTrancheShmemData *)
+		ShmemInitStruct("LWLock tranches", sizeof(LWLockTrancheShmemData), &found);
+	if (!found)
 	{
-		/* Allocate space for LWLockTranches */
-		LWLockTranches = (LWLockTrancheShmemData *)
-			ShmemAlloc(sizeof(LWLockTrancheShmemData));
+		/* Calculate total number of locks needed in the main array */
+		LWLockTranches->num_main_array_locks =
+			NUM_FIXED_LWLOCKS + NumLWLocksForNamedTranches();
 
 		/* Initialize the dynamic-allocation counter for tranches */
-		SpinLockInit(&LWLockTranches->lock);
 		LWLockTranches->num_user_defined = 0;
 
-		/* Allocate and initialize the main array */
-		numLocks = NUM_FIXED_LWLOCKS + NumLWLocksForNamedTranches();
-		MainLWLockArray = (LWLockPadded *) ShmemAlloc(numLocks * sizeof(LWLockPadded));
+		SpinLockInit(&LWLockTranches->lock);
+	}
+
+	/* Allocate and initialize the main array */
+	numLocks = LWLockTranches->num_main_array_locks;
+	MainLWLockArray = (LWLockPadded *)
+		ShmemInitStruct("Main LWLock array", numLocks * sizeof(LWLockPadded), &found);
+	if (!found)
+	{
+		/* Initialize all LWLocks */
 		InitializeLWLocks(numLocks);
 	}
 }
diff --git a/src/include/storage/lwlock.h b/src/include/storage/lwlock.h
index 30557631eb8..61f0dbe749a 100644
--- a/src/include/storage/lwlock.h
+++ b/src/include/storage/lwlock.h
@@ -73,11 +73,6 @@ typedef union LWLockPadded
 
 extern PGDLLIMPORT LWLockPadded *MainLWLockArray;
 
-/* forward declaration of private type for use only by lwlock.c */
-typedef struct LWLockTrancheShmemData LWLockTrancheShmemData;
-
-extern PGDLLIMPORT LWLockTrancheShmemData *LWLockTranches;
-
 /*
  * It's a bit odd to declare NUM_BUFFER_PARTITIONS and NUM_LOCK_PARTITIONS
  * here, but we need them to figure out offsets within MainLWLockArray, and
@@ -132,7 +127,7 @@ extern bool LWLockWaitForVar(LWLock *lock, pg_atomic_uint64 *valptr, uint64 oldv
 extern void LWLockUpdateVar(LWLock *lock, pg_atomic_uint64 *valptr, uint64 val);
 
 extern Size LWLockShmemSize(void);
-extern void CreateLWLocks(void);
+extern void LWLockShmemInit(void);
 extern void InitLWLockAccess(void);
 
 extern const char *GetLWLockIdentifier(uint32 classId, uint16 eventId);