From: Dave Chinner <david@fromorbit.com>
Subject: Re: [URGENT PATCH] ext4: fix potential deadlock in ext4_evict_inode()
Date: Fri, 26 Aug 2011 19:24:26 +1000
Message-ID: <20110826092426.GB3162@dastard>
References: <E1QwnAu-00087H-8X@tytso-glaptop.cam.corp.google.com>
 <20110826073507.GZ3162@dastard>
 <20110826084403.GA3162@dastard>
 <4E576152.9060405@tao.ma>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Cc: Theodore Ts'o <tytso@mit.edu>, Jiaying Zhang <jiayingz@google.com>,
	linux-ext4@vger.kernel.org
To: Tao Ma <tm@tao.ma>
Content-Disposition: inline
In-Reply-To: <4E576152.9060405@tao.ma>
Sender: linux-ext4-owner@vger.kernel.org

On Fri, Aug 26, 2011 at 05:03:14PM +0800, Tao Ma wrote:
> On 08/26/2011 04:44 PM, Dave Chinner wrote:
> > On Fri, Aug 26, 2011 at 05:35:07PM +1000, Dave Chinner wrote:
> >> On Thu, Aug 25, 2011 at 11:33:44PM -0400, Theodore Ts'o wrote:
> >>>
> >>> Note: this will probably need to be sent to Linus as an emergency
> >>> bugfix ASAP, since it was introduced in 3.1-rc1, so it represents a
> >>> regression.
> >>
> >> It doesn't appear to be a bug. All of the new ext4 lockdep reports
> >> in 3.1 I've seen (except for the mmap_sem/i_mutex one) are false
> >> positives....
> > 
> > While the lockdep report is false positive, I agree that your
> > change is the right fix to make - the IO completions are already
> > queued on the workqueue, so they don't need to be flushed to get
> > them to complete. All that needs to be done is call
> > ext4_ioend_wait() for them to complete, and that gets rid of the
> > i_mutex altogether. (*)
> ext4_ioend_wait can't work here for a nasty bug. Please see the commit
> log of 2581fdc8.

Unless I'm missing something, the described race with
ext4_truncate() flushing completions without the i_mutex lock held
cannot occur if you've already waited for all pending completions to
drain by calling ext4_ioend_wait()....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com