[PATCH 4/4] change_mnt_propagation(): calculate propagation source only if we'll need it

[Al Viro <viro@zeniv.linux.org.uk>]

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: 955336e204ab "do_make_slave(): choose new master sanely"
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
 fs/pnode.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/pnode.c b/fs/pnode.c
index 1c789f88b3d2..6f7d02f3fa98 100644
--- a/fs/pnode.c
+++ b/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 {
-- 
2.47.2