Subject: Re: circular locking dependency detected panic in filldir when
 CONFIG_PROVE_LOCKING=y
From: Trond Myklebust <trond.myklebust@fys.uio.no>
To: Joe Korty <joe.korty@ccur.com>, Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Al Viro <viro@zeniv.linux.org.uk>, Steve French <sfrench@us.ibm.com>,
       LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@elte.hu>
In-Reply-To: <20090918211550.GA8258@tsunami.ccur.com>
References: <20090918211550.GA8258@tsunami.ccur.com>
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Date: Fri, 18 Sep 2009 17:39:01 -0400
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On Fri, 2009-09-18 at 17:15 -0400, Joe Korty wrote:
> I experienced a might_fault panic from NFS's use of filldir
> in a 2.6.31 kernel compiled with CONFIG_PROVE_LOCKING=y.
> 
> Looking at filldir, I see it is accessing user space with
> __put_dir's (which are inatomic) and with one copy_to_user
> (which is not inatomic).  It is the single copy_to_user
> which is causing the might_fault panic.
> 
> It doesn't make any sense to be mixing use of inatomic
> and non-inatomic services in filldir.  Either all should be
> the inatomic version, or none should be.
> 
> The might_fault condition being reported by the panic looks
> real to me, so I suspect the wrong answer is converting
> everything to the inatomic version, since that just
> suppresses the circular dependency check while leaving
> the circular dependency in place.

Yes. This is known... Please see 
   http://thread.gmane.org/gmane.linux.nfs/28578
and
   http://thread.gmane.org/gmane.linux.nfs/27406

I'm still hoping the VM folks can see fit to merge Peter's fix at some
point...

Cheers
  Trond

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