linux-kernel - Re: btrfs/mmap lockdep report from 3.2

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Date:	Wed, 18 Jan 2012 00:25:50 +0100
From:	Jan Kara <jack@...e.cz>
To:	Chris Mason <chris.mason@...cle.com>
Cc:	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: btrfs/mmap lockdep report from 3.2

On Mon 09-01-12 20:44:32, Chris Mason wrote:
> On Mon, Jan 09, 2012 at 08:03:32PM -0500, Dave Jones wrote:
> > Just hit this with Linus' tree as of 37cfc3f67db9f2d907f6bfcfae590cdbbef623e8
> > 
> > 	Dave
> > 
> > [  130.932837] ======================================================
> > [  130.933032] [ INFO: possible circular locking dependency detected ]
> > [  130.933032] 3.2.0+ #19 Not tainted
> > [  130.933032] -------------------------------------------------------
> > [  130.933032] firefox/1630 is trying to acquire lock:
> > [  130.933032]  (&sb->s_type->i_mutex_key#13){+.+.+.}, at: [<ffffffffa011fd5b>] btrfs_page_mkwrite+0x5b/0x310 [btrfs]
> > [  130.933032] 
> > [  130.933032] but task is already holding lock:
> > [  130.933032]  (&mm->mmap_sem){++++++}, at: [<ffffffff816a38c8>] do_page_fault+0xe8/0x5d0
> > [  130.933032] 
> > [  130.933032] which lock already depends on the new lock.
> 
> From a lockdep point of view, this is real.  But the page_mkwrite
> code is never called on directories so it is actually safe.  We'll fix
> it up.
  Well, but hitting a fault (and thus acquiring mmap_sem) while holding
i_mutex can happen also for normal files when we copy data from userspace.
So I believe the deadlock is real...

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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