[PATCH] vfs: Speed up deactivate_super for non-modular filesystems

[ebiederm@xmission.com (Eric W. Biederman)]

Recently it was observed that a distilled version of vsftp was taking a
surprising amount of time reaping zombies.  A measurement was taken
and vsftp was taking about 4ms (one jiffie) to reap each zombie and
those 4ms were spent spleeping in rcu_barrier in deactivate_locked_super.

The reason vsftp was sleeping in deactivate_locked_super is because
vsftp creates a pid namespace for each connection, and with that
pid namespace comes an internal mount of /proc.  That internal mount
of proc is unmounted when the last process in the pid namespace is
reaped.

/proc and similar non-modular filesystems do not need a rcu_barrier
in deactivate_locked_super.  Being non-modular there is no danger
of the rcu callback running after the module is unloaded.

Therefore do the easy thing and remove 4ms+ from unmount times by only
calling rcu_barrier for modular filesystems in unmount.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
---
 fs/super.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index cf00177..c739ef8 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -261,7 +261,8 @@ void deactivate_locked_super(struct super_block *s)
 		 * We need to call rcu_barrier so all the delayed rcu free
 		 * inodes are flushed before we release the fs module.
 		 */
-		rcu_barrier();
+		if (fs->owner)
+			rcu_barrier();
 		put_filesystem(fs);
 		put_super(s);
 	} else {
-- 
1.7.5.4