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Re: [dm-devel] [PATCH] Fix deadlock with request based dm and some drivers



(Cc to dm-devel)

On 11/06/12 20:29, Jens Axboe wrote:
> I recently fixed a deadlock for a customer we have with
> the below patch. I have queued it up in my tree as not
> to lose it. Can I have an ack from you, or do you want
> to submit it yourself? I've marked it stable as well.

Could you tell details about how the deadlock happened?

dm's end_io always throws the completion handling to softirq:
  end_clone_request()
    dm_complete_request()
      blk_complete_request()

then it is processed in softirq context:
  dm_softirq_done()
    dm_done()
      dm_end_request()
        rq_completed(run_queue=true)
          blk_run_queue()

Since queue_lock is always held with interrupt disabled,
I couldn't see why it could deadlock.

Request-based dm followed the completion model of scsi
mid layer. So similar code path exists in scsi, too.
For example:
  scsi_request_fn()
    scsi_kill_request()
      blk_complete_request()
then:
  scsi_softirq_done()
    scsi_io_completion()
      scsi_next_command()
        scsi_run_queue()
          spin_lock queue_lock
          __blk_run_queue()

If calling run-queue from softirq_done_fn() can cause deadlock,
I'm afraid the problem is not limited to dm.

> From 8ca211056519ac06bc96fb134dca1f8eb2141407 Mon Sep 17 00:00:00 2001
> From: Jens Axboe <axboe kernel dk>
> Date: Tue, 6 Nov 2012 12:24:26 +0100
> Subject: [PATCH] dm: fix deadlock with request based dm and queue request_fn
>  recursion
> 
> Request based dm attempts to re-run the request queue off the
> request completion path. If used with a driver that potentially does
> end_io from its request_fn, we could deadlock trying to recurse
> back into request dispatch. Fix this by punting the request queue
> run to kblockd.
> 
> Tested to fix a quickly reproducible deadlock in such a scenario.
> 
> Cc: stable kernel org
> Signed-off-by: Jens Axboe <axboe kernel dk>
> ---
>  drivers/md/dm.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> index 02db918..77e6eff 100644
> --- a/drivers/md/dm.c
> +++ b/drivers/md/dm.c
> @@ -740,8 +740,14 @@ static void rq_completed(struct mapped_device *md, int rw, int run_queue)
>  	if (!md_in_flight(md))
>  		wake_up(&md->wait);
>  
> +	/*
> +	 * Run this off this callpath, as drivers could invoke end_io while
> +	 * inside their request_fn (and holding the queue lock). Calling
> +	 * back into ->request_fn() could deadlock attempting to grab the
> +	 * queue lock again.
> +	 */
>  	if (run_queue)
> -		blk_run_queue(md->queue);
> +		blk_run_queue_async(md->queue);
>  
>  	/*
>  	 * dm_put() must be at the end of this function. See the comment above


-- 
Jun'ichi Nomura, NEC Corporation


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