Convert unloggedLSN to an atomic variable.

Currently, this variable is an XLogRecPtr protected by a spinlock.
By converting it to an atomic variable, we can remove the spinlock,
which saves a small amount of shared memory space.  Since this code
is not performance-critical, we use atomic operations with full
barrier semantics to make it easy to reason about correctness.

Author: John Morris
Reviewed-by: Michael Paquier, Robert Haas, Andres Freund, Stephen Frost, Bharath Rupireddy
Discussion: https://postgr.es/m/BYAPR13MB26772534335255E50318C574A0409%40BYAPR13MB2677.namprd13.prod.outlook.com
Discussion: https://postgr.es/m/MN2PR13MB2688FD8B757316CB5C54C8A2A0DDA%40MN2PR13MB2688.namprd13.prod.outlook.com

src/backend/access/transam/xlog.c

@@ -470,9 +470,8 @@ typedef struct XLogCtlData
 
 	XLogSegNo	lastRemovedSegNo;	/* latest removed/recycled XLOG segment */
 
-	/* Fake LSN counter, for unlogged relations. Protected by ulsn_lck. */
-	XLogRecPtr	unloggedLSN;
-	slock_t		ulsn_lck;
+	/* Fake LSN counter, for unlogged relations. */
+	pg_atomic_uint64 unloggedLSN;
 
 	/* Time and LSN of last xlog segment switch. Protected by WALWriteLock. */
 	pg_time_t	lastSegSwitchTime;
@@ -4498,14 +4497,7 @@ DataChecksumsEnabled(void)
 XLogRecPtr
 GetFakeLSNForUnloggedRel(void)
 {
-	XLogRecPtr	nextUnloggedLSN;
-
-	/* increment the unloggedLSN counter, need SpinLock */
-	SpinLockAcquire(&XLogCtl->ulsn_lck);
-	nextUnloggedLSN = XLogCtl->unloggedLSN++;
-	SpinLockRelease(&XLogCtl->ulsn_lck);
-
-	return nextUnloggedLSN;
+	return pg_atomic_fetch_add_u64(&XLogCtl->unloggedLSN, 1);
 }
 
 /*
@@ -4921,7 +4913,7 @@ XLOGShmemInit(void)
 
 	SpinLockInit(&XLogCtl->Insert.insertpos_lck);
 	SpinLockInit(&XLogCtl->info_lck);
-	SpinLockInit(&XLogCtl->ulsn_lck);
+	pg_atomic_init_u64(&XLogCtl->unloggedLSN, InvalidXLogRecPtr);
 }
 
 /*
@@ -5526,9 +5518,11 @@ StartupXLOG(void)
 	 * the unlogged LSN counter can be reset too.
 	 */
 	if (ControlFile->state == DB_SHUTDOWNED)
-		XLogCtl->unloggedLSN = ControlFile->unloggedLSN;
+		pg_atomic_write_membarrier_u64(&XLogCtl->unloggedLSN,
+									   ControlFile->unloggedLSN);
 	else
-		XLogCtl->unloggedLSN = FirstNormalUnloggedLSN;
+		pg_atomic_write_membarrier_u64(&XLogCtl->unloggedLSN,
+									   FirstNormalUnloggedLSN);
 
 	/*
 	 * Copy any missing timeline history files between 'now' and the recovery
@@ -7110,9 +7104,7 @@ CreateCheckPoint(int flags)
 	 * unused on non-shutdown checkpoints, but seems useful to store it always
 	 * for debugging purposes.
 	 */
-	SpinLockAcquire(&XLogCtl->ulsn_lck);
-	ControlFile->unloggedLSN = XLogCtl->unloggedLSN;
-	SpinLockRelease(&XLogCtl->ulsn_lck);
+	ControlFile->unloggedLSN = pg_atomic_read_membarrier_u64(&XLogCtl->unloggedLSN);
 
 	UpdateControlFile();
 	LWLockRelease(ControlFileLock);