On Mon, Jan 13, 2020 at 02:03:00PM +0800, Ian Kent wrote:

Oh wait, for systemd I was actually looking at:
https://github.com/systemd/systemd/blob/master/src/shared/switch-root.c

quoted

Mind you, that's not the actual systemd repo. either I probably
need to look a lot deeper (and at the actual systemd repo) to
work out what's actually being called.

quoted

Sigh...  Guess I'll have to dig that Fedora KVM image out and
try to see what it's about... ;-/  Here comes a couple of hours
of build...

D'oh...  And yes, that would've been a bisect hazard - switch to
path_lookupat() later in the series gets rid of that.  Incremental
(to be foldede, of course):

diff --git a/fs/namei.c b/fs/namei.c
index 1793661c3342..204677c37751 100644
--- a/fs/namei.c
+++ b/fs/namei.c

@@ -2634,7 +2634,7 @@ path_mountpoint(struct nameidata *nd, unsigned flags, struct path *path)
 		(err = lookup_last(nd)) > 0) {
 		s = trailing_symlink(nd);
 	}
-	if (!err)
+	if (!err && (nd->flags & LOOKUP_RCU))
 		err = unlazy_walk(nd);
 	if (!err)
 		err = handle_lookup_down(nd);