pg_start_backup() can use a share lock to lock ControlFileLock

instead of an exclusive lock.

The change is almost for code cleanup. Since there seems to be no
performance benefits from it, backports should not be needed.

Fujii Masao

src/backend/access/transam/xlog.c

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2010, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.379 2010/02/26 02:00:35 momjian Exp $
+ * $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.380 2010/03/10 02:04:48 itagaki Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -7897,7 +7897,7 @@ pg_start_backup(PG_FUNCTION_ARGS)
 		 * REDO pointer.  The oldest point in WAL that would be needed to
 		 * restore starting from the checkpoint is precisely the REDO pointer.
 		 */
-		LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
+		LWLockAcquire(ControlFileLock, LW_SHARED);
 		checkpointloc = ControlFile->checkPoint;
 		startpoint = ControlFile->checkPointCopy.redo;
 		LWLockRelease(ControlFileLock);