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

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Date:	Thu, 27 Jul 2006 20:02:46 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Anton Altaparmakov <aia21@....ac.uk>
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

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.

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

-- 
SUSE Labs, Novell Inc.
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