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

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Message-ID: <20120112201502.GA15190@kroah.com>
Date:	Thu, 12 Jan 2012 12:15:02 -0800
From:	Greg KH <greg@...ah.com>
To:	Paul Taysom <taysom@...omium.org>
Cc:	Paul Taysom <taysom@...gle.com>, Mandeep Baines <msb@...omium.org>,
	Jens Axboe <axboe@...nel.dk>, Theodore Tso <tytso@...gle.com>,
	Andrew Morton <akpm@...gle.com>, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fs: Fix mod_timer crash when removing USB sticks

On Thu, Jan 12, 2012 at 11:57:42AM -0800, Paul Taysom wrote:
> From: Paul Taysom <taysom@...gle.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.
> 
> Signed-off-by: Paul Taysom <taysom@...omium.org>
> Downstream-bug-report: http://crosbug.com/24165
> Cc: Mandeep Baines <msb@...omium.org>
> Cc: Greg KH <greg@...ah.com>
> Cc: Jens Axboe <axboe@...nel.dk>
> Cc: Theodore Tso <tytso@...gle.com>
> Cc: Andrew Morton <akpm@...gle.com>
> Cc: <linux-usb@...r.kernel.org>
> Cc: <linux-kernel@...r.kernel.org>
> Cc: Alexander Viro <viro@...iv.linux.org.uk>
> Cc: <linux-fsdevel@...r.kernel.org>
> ---
>  fs/block_dev.c |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index afe74dd..9f9b617 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -1,4 +1,4 @@
> -/*
> +nvalid/*
>   *  linux/fs/block_dev.c
>   *
>   *  Copyright (C) 1991, 1992  Linus Torvalds

Minor nit, I don't think you ment the first line of the file to be
changed this way....

greg k-h
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