diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index f13f9c1..c4bc332 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -505,10 +505,13 @@ typedef struct XLogCtlInsert
 	 * of streaming base backups currently in progress. forcePageWrites is set
 	 * to true when either of these is non-zero. lastBackupStart is the latest
 	 * checkpoint redo location used as a starting point for an online backup.
+	 * exclusiveBackupStarted is true while pg_start_backup() is being called
+	 * during an exclusive backup.
 	 */
 	bool		exclusiveBackup;
 	int			nonExclusiveBackups;
 	XLogRecPtr	lastBackupStart;
+	bool		exclusiveBackupStarted;
 
 	/*
 	 * WAL insertion locks.
@@ -9833,7 +9836,8 @@ do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
 	WALInsertLockAcquireExclusive();
 	if (exclusive)
 	{
-		if (XLogCtl->Insert.exclusiveBackup)
+		if (XLogCtl->Insert.exclusiveBackup ||
+			XLogCtl->Insert.exclusiveBackupStarted)
 		{
 			WALInsertLockRelease();
 			ereport(ERROR,
@@ -9842,6 +9846,7 @@ do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
 					 errhint("Run pg_stop_backup() and try again.")));
 		}
 		XLogCtl->Insert.exclusiveBackup = true;
+		XLogCtl->Insert.exclusiveBackupStarted = true;
 	}
 	else
 		XLogCtl->Insert.nonExclusiveBackups++;
@@ -10178,6 +10183,16 @@ do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
 	PG_END_ENSURE_ERROR_CLEANUP(pg_start_backup_callback, (Datum) BoolGetDatum(exclusive));
 
 	/*
+	 * Mark that start phase has correctly finished for an exclusive backup.
+	 */
+	if (exclusive)
+	{
+		WALInsertLockAcquireExclusive();
+		XLogCtl->Insert.exclusiveBackupStarted = false;
+		WALInsertLockRelease();
+	}
+
+	/*
 	 * We're done.  As a convenience, return the starting WAL location.
 	 */
 	if (starttli_p)
@@ -10195,8 +10210,10 @@ pg_start_backup_callback(int code, Datum arg)
 	WALInsertLockAcquireExclusive();
 	if (exclusive)
 	{
-		Assert(XLogCtl->Insert.exclusiveBackup);
+		Assert(XLogCtl->Insert.exclusiveBackup &&
+			   XLogCtl->Insert.exclusiveBackupStarted);
 		XLogCtl->Insert.exclusiveBackup = false;
+		XLogCtl->Insert.exclusiveBackupStarted = false;
 	}
 	else
 	{
@@ -10287,6 +10304,13 @@ do_pg_stop_backup(char *labelfile, bool waitforarchive, TimeLineID *stoptli_p)
 					(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 					 errmsg("exclusive backup not in progress")));
 		}
+		if (XLogCtl->Insert.exclusiveBackupStarted)
+		{
+			WALInsertLockRelease();
+			ereport(ERROR,
+					(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+					 errmsg("exclusive backup currently starting")));
+		}
 		XLogCtl->Insert.exclusiveBackup = false;
 	}
 	else