change_mnt_propagation(): calculate propagation source only if we'll need it

We only need it when mount in question was sending events downstream (then
recepients need to switch to new master) or the mount is being turned into
slave (then we need a new master for it).

That wouldn't be a big deal, except that it causes quite a bit of work
when umount_tree() is taking a large peer group out.  Adding a trivial
"don't bother calling propagation_source() unless we are going to use
its results" logics improves the things quite a bit.

We are still doing unnecessary work on bulk removals from propagation graph,
but the full solution for that will have to wait for the next merge window.

Fixes: 955336e "do_make_slave(): choose new master sanely"
Reviewed-by: Christian Brauner <[email protected]>
Signed-off-by: Al Viro <[email protected]>

Author: Al Viro

fs/pnode.c

@@ -111,7 +111,8 @@ void change_mnt_propagation(struct mount *mnt, int type)
 		return;
 	}
 	if (IS_MNT_SHARED(mnt)) {
-		m = propagation_source(mnt);
+		if (type == MS_SLAVE || !hlist_empty(&mnt->mnt_slave_list))
+			m = propagation_source(mnt);
 		if (list_empty(&mnt->mnt_share)) {
 			mnt_release_group_id(mnt);
 		} else {