linux-kernel - Re: [PATCH 1/7] vfs - merge path_is_mountpoint() and path_is_mountpoint

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Message-ID: <20161203051322.GA24765@ZenIV.linux.org.uk>
Date:   Sat, 3 Dec 2016 05:13:22 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Ian Kent <raven@...maw.net>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        autofs mailing list <autofs@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Omar Sandoval <osandov@...ndov.com>
Subject: Re: [PATCH 1/7] vfs - merge path_is_mountpoint() and
 path_is_mountpoint_rcu()

	FWIW, I've folded that pile into vfs.git#work.autofs.

Problems:
	* (fixed) __path_is_mountpoint() should _not_ treat NULL from
__lookup_mnt() as "nothing's mounted there" until it has checked
that mount_lock hadn't been touched - mount --move on something unrelated
can race with lockless hash lookup and lead to false negatives.
	* linux/mount.h might be the wrong place for path_is_mountpoint().
Or it shouldn't be inlined.  I don't like the includes you've added there.
	* path_has_submounts() is broken.  At the very least, it's
AB-BA between mount_lock and rename_lock.  I would suggest trying to
put read_seqlock_excl(&mount_lock) around the call of d_walk() in there,
and using __lookup_mnt() in the callback (without retries on the mount_lock,
of course - read_seqlock_excl done on the outside is enough).  I'm not sure
if it won't cause trouble with contention, though; that needs testing.  As
it is, that function is broken in #work.autofs, same as it is in -mm and
-next.
	* the last one (propagation-related) is too ugly to live - at the
very least, its pieces should live in fs/pnode.c; exposing propagate_next()
is simply wrong.  I hadn't picked that one at all, and I would suggest
coordinating anything in that area with ebiederman - he has some work
around fs/pnode.c and you risk stepping on his toes.