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Re: [PATCH 2/2] blkdev: check for valid request queue before issuing flu

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 2/2] blkdev: check for valid request queue before issuing flush
From: Jens Axboe <axboe@xxxxxxxxx>
Date: Tue, 13 Jul 2010 14:55:14 +0200
Cc: xfs@xxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx
In-reply-to: <1279007450-10457-3-git-send-email-david@xxxxxxxxxxxxx>
References: <1279007450-10457-1-git-send-email-david@xxxxxxxxxxxxx> <1279007450-10457-3-git-send-email-david@xxxxxxxxxxxxx>
On 2010-07-13 09:50, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
> 
> Issuing a blkdev_issue_flush() on an unconfigured loop device causes a panic 
> as
> q->make_request_fn is not configured. This can occur when trying to mount the
> unconfigured loop device as an XFS filesystem. There are no guards that catch
> the bio before the request function is called because we don't add a payload 
> to
> the bio. Instead, manually check this case as soon as we have a pointer to the
> queue to flush.
> 
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> ---
>  block/blk-barrier.c |    9 +++++++++
>  1 files changed, 9 insertions(+), 0 deletions(-)
> 
> diff --git a/block/blk-barrier.c b/block/blk-barrier.c
> index 0d710c9..0fd766e 100644
> --- a/block/blk-barrier.c
> +++ b/block/blk-barrier.c
> @@ -319,6 +319,15 @@ int blkdev_issue_flush(struct block_device *bdev, gfp_t 
> gfp_mask,
>       if (!q)
>               return -ENXIO;
>  
> +     /*
> +      * some block devices may not have their queue correctly set up here
> +      * (e.g. loop device without a backing file) and so issuing a flush
> +      * here will panic. Ensure there is a request function before issuing
> +      * the barrier.
> +      */
> +     if (!q->make_request_fn)
> +             return -ENXIO;
> +
>       bio = bio_alloc(gfp_mask, 0);
>       bio->bi_end_io = bio_end_empty_barrier;
>       bio->bi_bdev = bdev;

This may appear ugly, but I think the patch is fine since there's not
much we can do about the loop crap (outside of changing how you do
setup/configure of it).

I'll apply this, thanks.

-- 
Jens Axboe

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