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

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Message-ID: <20120110014432.GC26546@shiny>
Date:	Mon, 9 Jan 2012 20:44:32 -0500
From:	Chris Mason <chris.mason@...cle.com>
To:	Dave Jones <davej@...hat.com>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: btrfs/mmap lockdep report from 3.2

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.

-chris
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