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 is 8 seconds to mount and 0.6 to unmount. I think this 200-fold speed up is worth the slightly higher system impact of use synchronize_rcu_expedited(). Signed-off-by: NeilBrown --- Cc: to Paul and Josh in case they'll correct me if using _expedited() is really bad here. Thanks, NeilBrown fs/namespace.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/fs/namespace.c b/fs/namespace.c index 3b601f115b6c..fce91c447fab 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -1420,7 +1420,7 @@ 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