linux-kernel - Re: VFS deadlock ?

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Message-ID: <CA+55aFwd=oTFEfryRs6FgDDHmcw1TM=YkkiDrcWrz-zuFpB26w@mail.gmail.com>
Date:	Thu, 21 Mar 2013 17:15:12 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
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 5:08 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> 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...

Hmm. But again, that can't actually happen here. We're in /proc. You
can't move the entries around. Also, we only changed the locking order
for the "inode is identical" case where we take only *one* lock, we
didn't change it for the cases where we take multiple locks (and order
them topologically).

So I agree that we need to avoid aliased directories in the *general*
case. I'm just arguing that for the specific case of /proc, we should
be ok. No?

             Linus
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