From mboxrd@z Thu Jan  1 00:00:00 1970
From: Al Viro <viro-3bDd1+5oDREiFSDQTTA3OLVCufUGDwFn@public.gmane.org>
Subject: Re: [PATCH review 11/11] mnt: Honor MNT_LOCKED when detaching mounts
Date: Fri, 9 Jan 2015 22:17:15 +0000
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Cc: Andrey Vagin <avagin-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org>, Richard Weinberger <richard-/L3Ra7n9ekc@public.gmane.org>,
	Linux Containers <containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org>,
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	Linus Torvalds <torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
To: "Eric W. Biederman" <ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
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On Fri, Jan 09, 2015 at 03:30:10PM -0600, Eric W. Biederman wrote:

> Where does the rcu_read_lock() happen?
> I assume this is kill from fs_pin.kill?

This is what I'd rather have *calling* fs_pin.kill (the ... part in there
being the callback, with wakeup done as part of pin_remove(), which would
be called by ->kill()).  The thing is, I don't want mnt_pin_kill() et.al.
to grab refcount on fs_pin (or for the refcount to be necessary there).

IOW, any refcounting belong on the same level as ->kill() implementation
itself; for ex-mountpoint-related ones we'd need none whatsoever (->kill()
would do
	dput(ex-mountpoint dentry);
	pin_remove(pin);
	mntput_no_expire(containing struct mount);
and that would be it), for kernel/acct.c ones we do need some refcounting,
but only for the local reasons - note that it's playing directly with
refcount anyway, which is a pretty clear indication that we'd be better off
with fs/fs_pin.c _not_ messing with that refcount in the first place.
Getting rid of pin_put() also wouldn't hurt.