linux-kernel - Re: VFS deadlock ?

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Message-ID: <20130322000803.GG21522@ZenIV.linux.org.uk>
Date:	Fri, 22 Mar 2013 00:08:03 +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 04:58:41PM -0700, Linus Torvalds wrote:

> And the only other reason we don't want to allow it is to make sure
> you can't have directory loops etc, afaik, and again, for this
> particular case of /proc, we happen to be ok.

Not really.  Do that and yes, this deadlock goes away.  But the locking
order in general goes to hell - we order directory inodes by "which dentry
is an ancestor of another?"  So we have no warranty that we won't get
alias1/foo/bar/baz < alias2/foo.  Take rename_lock() on those two and
have it race with rmdir alias2/foo/bar/baz (locks alias2/foo/bar, then
alias2/foo/bar/baz) and rmdir alias2/foo/bar (locks alias2/foo and
alias2/foo/bar).  Oops - we have a cycle now...
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