linux-kernel - Re: [git pull] vfs and fs fixes

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Date:	Tue, 17 Apr 2012 13:06:14 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Al Viro <viro@...iv.linux.org.uk>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [git pull] vfs and fs fixes

On Tue, Apr 17, 2012 at 09:33:06AM -0700, Linus Torvalds wrote:
> On Tue, Apr 17, 2012 at 9:22 AM, J. Bruce Fields <bfields@...ldses.org> wrote:
> >
> > Ugh, no, I think you're right:
> >
> >        rename A/a->A/b
> >        rename B/b->B/b
> >
> > where A/a and B/a are the same file, and A/b and B/b are the same file,
> > can result in the first rename holding the lock on A and a and waiting
> > on b, and the second holding the lock on B and b and waiting on a.
> 
> In fact I don't think you need even that much. Just a simple
> 
>   touch a
>   ln a b
>   mv a b
> 
> looks like it should deadlock on itself, no? source and dest inodes
> will be the same, so the mutex_lock() will just deadlock without even
> any ABBA race.
> 
> (I didn't really check - maybe there is some reason that doesn't happen).

Yeah, rename has that funny exception that makes the above a no-op, so I
think that's safe.

But the patch is still wrong; back to the drawing board.

Maybe a paper bag over my head will help my concentration....

--b.
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