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

commit 41abc4e1af369bb5438eaee398e3beee690cc8ca upstream.

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 <hare@suse.de>
Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index a562d5a4fa9d8322c7e406f978054d7931d697bb..2f03e8e10c24ef7cc9475d042939696351c7541f 100644 (file)
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -2253,7 +2253,8 @@ static struct dm_table *__bind(struct mapped_device *md, struct dm_table *t,
                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;
 }
@@ -2694,7 +2695,8 @@ int dm_suspend(struct mapped_device *md, unsigned suspend_flags)
         * 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
@@ -2714,7 +2716,8 @@ int dm_suspend(struct mapped_device *md, unsigned suspend_flags)
 
        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) {