linux-kernel - Re: VFS deadlock ?

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Message-ID: <20130322001257.GH21522@ZenIV.linux.org.uk>
Date:	Fri, 22 Mar 2013 00:12:57 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: VFS deadlock ?

On Thu, Mar 21, 2013 at 05:01:49PM -0700, Linus Torvalds wrote:
> On Thu, Mar 21, 2013 at 4:58 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > So yes, it's against the rules, and we get that deadlock right now,
> > but one solution would be to just allow this particular case. The
> > patch for the deadlock looks dead simple:
> 
> It should go without saying that that whitespace-damaged patch is
> entirely untested. But especially since we need to back-port whatever
> fix, it would be good if we could make the fix be something as simple
> as this. Because I don't believe we really want to backport some big
> network-namespace reorganization.
> 
> This is, of course, all assuming that hardlinked directories are ok if
> we can just guarantee the absence of loops. If there's some other
> reason why they'd be problematic, we're screwed.

See the posting upthread.  We could try to kludge around that as well
(e.g. have d_ancestor() compare ->d_inode instead of dentries themselves),
but I really think it's a lousy idea only inviting further abuse.

What we should do, IMO, is to turn /proc/<pid>/net into a honest symlink -
to ../nets/<netns ID>/net.  Hell, might even make it a magical symlink
instead...
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