linux-kernel - Re: [BUG?] possible recursive locking detected

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Message-Id: <1154003434.21849.44.camel@imp.csi.cam.ac.uk>
Date:	Thu, 27 Jul 2006 13:30:34 +0100
From:	Anton Altaparmakov <aia21@....ac.uk>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Andrew Morton <akpm@...l.org>, eike-kernel@...tec.de,
	linux-kernel@...r.kernel.org, aia21@...tab.net
Subject: Re: [BUG?] possible recursive locking detected

On Thu, 2006-07-27 at 20:02 +1000, Nick Piggin wrote:
> Anton Altaparmakov wrote:
> > On Thu, 2006-07-27 at 19:18 +1000, Nick Piggin wrote:
> > 
> >>Anton Altaparmakov wrote:
> 
> >>>I beg to differ.  It is a bug.  You cannot reenter the file system when
> >>>the file system is trying to allocate memory.  Otherwise you can never
> >>>allocate memory with any locks held or you are bound to introduce an
> >>>A->B B->A deadlock somewhere.
> >>
> >>I don't think it is a bug in general. It really depends on the allocation:
> >>
> >>- If it is a path that might be required in order to writeout a page, then
> >>yes GFP_NOFS is going to help prevent deadlocks.
> >>
> >>- If it is a path where you'll take the same locks as page reclaim requires,
> >>then again GFP_NOFS is required.
> >>
> >>For NTFS case, it seems like holding i_mutex on the write path falls foul
> >>of the second problem. But I agree with Andrew that this is a critical case
> >>where we do have to enter the fs. GFP_NOFS is too big a hammer to use.
> >>
> >>I guess you'd have to change NTFS to do something sane privately, or come
> >>up with a nice general solution that doesn't harm the common filesystems
> >>that apparently don't have a problem here... can you just add GFP_NOFS to
> >>NTFS's mapping_gfp_mask to start with?
> > 
> > 
> > I don't think NTFS has a problem either.  It is a theoretical problem
> 
> No, I mean: *really* doesn't have a problem. If Andrew says ext2 doesn't
> need i_mutex in reclaim, then I tend to believe him.
> 
> > with an extremely small chance of being hit.  I am happy to have such a
> > problem for now.  There are more pressing problems to solve.  The only
> > thing that needs to happen is for the messages to stop so people stop
> > complaining / getting worried about them...
> 
> I guess the memory deadlock issue is probably mostly theoretical, although
> it is still nice to get them fixed. I'd imagine a deadlock condition -- if
> one really exists -- could be hit without much problem though. Page reclaim
> will readily get kicked from the write(2) path, and will potentially free
> *lots* of stuff from there.
> 
> If it isn't a problem for you, I'd suspect it might be due to some other
> conditions which happen to mean it is avoided. For example, the inode who's
> i_mutex you are holding will have a raised refcount AFAIK, so it will not
> get reclaimed and so may get around your problem.

That is true, yes.  So at least in that respect it should be safe.

> This would be a valid solution IMO. It probably could do with documentation
> to outline the issues, though.

That is true.

Best regards,

        Anton
-- 
Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
Unix Support, Computing Service, University of Cambridge, CB2 3QH, UK
Linux NTFS maintainer / IRC: #ntfs on irc.freenode.net
WWW: http://www.linux-ntfs.org/ & http://www-stu.christs.cam.ac.uk/~aia21/

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