dm: do not call dm_sync_table() when creating new devices

When creating new devices dm_sync_table() calls
synchronize_rcu_expedited(), causing _all_ pending RCU pointers to be
flushed. This causes a latency overhead that is especially noticeable
when creating lots of devices.

And all of this is pointless as there are no old maps to be
disconnected, and hence no stale pointers which would need to be
cleared up.

Signed-off-by: Hannes Reinecke <[e-mail]>
Reviewed-by: Mikulas Patocka <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm.c

···
 		set_bit(DMF_MERGE_IS_OPTIONAL, &md->flags);
 	else
 		clear_bit(DMF_MERGE_IS_OPTIONAL, &md->flags);
-	dm_sync_table(md);
+	if (old_map)
+		dm_sync_table(md);
 
 	return old_map;
 }
···
 	 * flush_workqueue(md->wq).
 	 */
 	set_bit(DMF_BLOCK_IO_FOR_SUSPEND, &md->flags);
-	synchronize_srcu(&md->io_barrier);
+	if (map)
+		synchronize_srcu(&md->io_barrier);
 
 	/*
 	 * Stop md->queue before flushing md->wq in case request-based
···
 
 	if (noflush)
 		clear_bit(DMF_NOFLUSH_SUSPENDING, &md->flags);
-	synchronize_srcu(&md->io_barrier);
+	if (map)
+		synchronize_srcu(&md->io_barrier);
 
 	/* were we interrupted ? */
 	if (r < 0) {