Re: [PATCH] fs: Fix mod_timer crash when removing USB sticks

From: Mandeep Singh Baines <msb@chromium.org>
To: Greg KH <greg@kroah.com>
Cc: Paul Taysom <taysom@chromium.org>,
	Paul Taysom <taysom@google.com>,
	Mandeep Baines <msb@chromium.org>,
	Olof Johansson <olofj@chromium.org>, Jens Axboe <axboe@kernel.dk>,
	Theodore Tso <tytso@google.com>, Andrew Morton <akpm@google.com>,
	linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH] fs: Fix mod_timer crash when removing USB sticks
Date: Thu, 12 Jan 2012 13:53:31 -0800	[thread overview]
Message-ID: <20120112215331.GB18166@google.com> (raw)
In-Reply-To: <20120112213830.GB17405@kroah.com>

Greg KH (greg@kroah.com) wrote:
> On Thu, Jan 12, 2012 at 01:15:35PM -0800, Paul Taysom wrote:
> > From: Paul Taysom <taysom@google.com>
> > 
> > A USB stick with a ext file system on it, would occasionally crash
> > when the stick was pulled.
> > 
> > The problem was a timer was being set on the Backing Device Interface,
> > bdi, after the USB device had been removed and the bdi had been
> > unregistered. The bdi would then be later reinitialized by zeroing
> > the timer without removing from the timer from the timer queue.
> > This would eventually result in a kernel crash (NULL ptr dereference).
> > 
> > When the bdi is unregistered, the dev field is set to NULL. This
> > indication is used by bdi_unregister to only unregister the device
> > once.
> > 
> > Fix: When the backing device is invalidated, the mapping backing_dev_info
> > should be redirected to the default_backing_dev_info.
> > 
> > Created 3 USB sticks with ext2, ext4 and one with both apple and DOS
> > file systems on it. Inserted and removed USB sticks many times in random
> > order. With out the bug fix, the kernel would soon crash. With the fix,
> > it did not. Ran on both stumpy and amd64-generic.
> > 
> > Change-Id: Icdd06cf3ced555dcd9994cfcc9478a9071a802f1
> 
> What is this field for?  It makes no sense for a kernel patch
> submission.
> 
> > Signed-off-by: Paul Taysom <taysom@chromium.org>
> > Downstream-bug-report: http://crosbug.com/24165
> 
> Is that a regular field that we now use?
> 

Hi Greg,

What is the conventional way of doing this? There is a lot of good
data in the bug report which might be useful to reviewers. We
couldn't find a de-facto way of referencing the downstream bug database
so we just made up a new field. Sorry. We'll use the correct
field name next time.

So what is the correct field name?

Regards,
Mandeep

> And shouldn't this go to the stable kernel releases as well?
> 
> Third time's a charm?
> 
> greg k-h