The synchronize_rcu() in namespace_unlock() is called every time
a filesystem is unmounted.  If a great many filesystems are mounted,
this can cause a noticable slow-down in, for example, system shutdown.

The sequence:
  mkdir -p /tmp/Mtest/{0..5000}
  time for i in /tmp/Mtest/*; do mount -t tmpfs tmpfs $i ; done
  time umount /tmp/Mtest/*

on a 4-cpu VM can report 8 seconds to mount the tmpfs filesystems, and
100 seconds to unmount them.

Boot the same VM with 1 CPU and it takes 18 seconds to mount the
tmpfs filesystems, but only 36 to unmount.

If we change the synchronize_rcu() to synchronize_rcu_expedited()
the umount time on a 4-cpu VM drop to 0.6 seconds

I think this 200-fold speed up is worth the slightly high system
impact of using synchronize_rcu_expedited().

Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> (from general rcu perspective)
Signed-off-by: NeilBrown <neilb@suse.com>
---

Al Viro says "I can live with this one" but this still hasn't landed.
Maybe someone else could apply it?

Thanks,
NeilBrown

Full quote from Al on 5th Oct:
> Umm...  IIRC, the last one got sidetracked on the other thing in the series...
> <checks> that was s_anon stuff.  I can live with this one; FWIW, what kind
> of load would trigger the impact of the change?  Paul?
which Paul replied to.

 fs/namespace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index a7f91265ea67..43a0d2c7449d 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1360,7 +1360,7 @@ static void namespace_unlock(void)
 	if (likely(hlist_empty(&head)))
 		return;
 
-	synchronize_rcu();
+	synchronize_rcu_expedited();
 
 	group_pin_kill(&head);
 }
-- 
2.14.0.rc0.dirty