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

[Greg KH <greg@kroah.com>]

On Thu, Jan 12, 2012 at 01:57:11PM -0800, Paul Taysom wrote:
> 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@chromium.org>
> Cc: Mandeep Baines <msb@chromium.org>
> Cc: Greg KH <greg@kroah.com>
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: Theodore Tso <tytso@google.com>
> Cc: Andrew Morton <akpm@google.com>
> Cc: <linux-usb@vger.kernel.org>
> Cc: <linux-kernel@vger.kernel.org>
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: <linux-fsdevel@vger.kernel.org>
> Cc: <stable@kernel.org>
> ---
>  fs/block_dev.c |    1 +
>  1 files changed, 1 insertions(+), 0 deletions(-)
> 
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index afe74dd..322cd05 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -110,6 +110,7 @@ void invalidate_bdev(struct block_device *bdev)
>  	 * But, for the strange corners, lets be cautious
>  	 */
>  	cleancache_flush_inode(mapping);
> +	mapping->backing_dev_info = &default_backing_dev_info;
>  }
>  EXPORT_SYMBOL(invalidate_bdev);

What ever happened to this patch?  Is it still needed?  Can you still
reproduce the problem on Linus's tree and older kernels?

thanks,

greg k-h