Re: [PATCH 08/10] block: make get_request[_wait]() fail if queue isdead

[Vivek Goyal]

On Tue, Oct 18, 2011 at 09:26:22PM -0700, Tejun Heo wrote:
> Currently get_request[_wait]() allocates request whether queue is dead
> or not.  This patch makes get_request[_wait]() return NULL if @q is
> dead.  blk_queue_bio() is updated to fail the submitted bio if request
> allocation fails.  While at it, add docbook comments for
> get_request[_wait]().
> 
> Note that the current code has rather unclear (there are spurious DEAD
> tests scattered around) assumption that the owner of a queue
> guarantees that no request travels block layer if the queue is dead
> and this patch in itself doesn't change much; however, this will allow
> fixing the broken assumption in the next patch.

Ok, so this patch will now make sure that __make_request() will return
error if queue is dead. So previous blk_throtl_bio() change is fine.

Thanks
Vivek

> 
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Jens Axboe <axboe@xxxxxxxxx>
> ---
>  block/blk-core.c |   54 ++++++++++++++++++++++++++++++++++++++----------------
>  1 files changed, 38 insertions(+), 16 deletions(-)
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 7a90317..fc10f53 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -710,10 +710,19 @@ static bool blk_rq_should_init_elevator(struct bio *bio)
>  	return true;
>  }
>  
> -/*
> - * Get a free request, queue_lock must be held.
> - * Returns NULL on failure, with queue_lock held.
> - * Returns !NULL on success, with queue_lock *not held*.
> +/**
> + * get_request - get a free request
> + * @q: request_queue to allocate request from
> + * @rw_flags: RW and SYNC flags
> + * @bio: bio to allocate request for (can be %NULL)
> + * @gfp_mask: allocation mask
> + *
> + * Get a free request from @q.  This function may fail under memory
> + * pressure or if @q is dead.
> + *
> + * Must be callled with @q->queue_lock held and,
> + * Returns %NULL on failure, with @q->queue_lock held.
> + * Returns !%NULL on success, with @q->queue_lock *not held*.
>   */
>  static struct request *get_request(struct request_queue *q, int rw_flags,
>  				   struct bio *bio, gfp_t gfp_mask)
> @@ -724,6 +733,9 @@ static struct request *get_request(struct request_queue *q, int rw_flags,
>  	const bool is_sync = rw_is_sync(rw_flags) != 0;
>  	int may_queue;
>  
> +	if (unlikely(test_bit(QUEUE_FLAG_DEAD, &q->queue_flags)))
> +		return NULL;
> +
>  	may_queue = elv_may_queue(q, rw_flags);
>  	if (may_queue == ELV_MQUEUE_NO)
>  		goto rq_starved;
> @@ -816,11 +828,18 @@ out:
>  	return rq;
>  }
>  
> -/*
> - * No available requests for this queue, wait for some requests to become
> - * available.
> +/**
> + * get_request_wait - get a free request with retry
> + * @q: request_queue to allocate request from
> + * @rw_flags: RW and SYNC flags
> + * @bio: bio to allocate request for (can be %NULL)
>   *
> - * Called with q->queue_lock held, and returns with it unlocked.
> + * Get a free request from @q.  This function keeps retrying under memory
> + * pressure and fails iff @q is dead.
> + *
> + * Must be callled with @q->queue_lock held and,
> + * Returns %NULL on failure, with @q->queue_lock held.
> + * Returns !%NULL on success, with @q->queue_lock *not held*.
>   */
>  static struct request *get_request_wait(struct request_queue *q, int rw_flags,
>  					struct bio *bio)
> @@ -834,6 +853,9 @@ static struct request *get_request_wait(struct request_queue *q, int rw_flags,
>  		struct io_context *ioc;
>  		struct request_list *rl = &q->rq;
>  
> +		if (unlikely(test_bit(QUEUE_FLAG_DEAD, &q->queue_flags)))
> +			return NULL;
> +
>  		prepare_to_wait_exclusive(&rl->wait[is_sync], &wait,
>  				TASK_UNINTERRUPTIBLE);
>  
> @@ -864,19 +886,15 @@ struct request *blk_get_request(struct request_queue *q, int rw, gfp_t gfp_mask)
>  {
>  	struct request *rq;
>  
> -	if (unlikely(test_bit(QUEUE_FLAG_DEAD, &q->queue_flags)))
> -		return NULL;
> -
>  	BUG_ON(rw != READ && rw != WRITE);
>  
>  	spin_lock_irq(q->queue_lock);
> -	if (gfp_mask & __GFP_WAIT) {
> +	if (gfp_mask & __GFP_WAIT)
>  		rq = get_request_wait(q, rw, NULL);
> -	} else {
> +	else
>  		rq = get_request(q, rw, NULL, gfp_mask);
> -		if (!rq)
> -			spin_unlock_irq(q->queue_lock);
> -	}
> +	if (!rq)
> +		spin_unlock_irq(q->queue_lock);
>  	/* q->queue_lock is unlocked at this point */
>  
>  	return rq;
> @@ -1300,6 +1318,10 @@ get_rq:
>  	 * Returns with the queue unlocked.
>  	 */
>  	req = get_request_wait(q, rw_flags, bio);
> +	if (unlikely(!req)) {
> +		bio_endio(bio, -ENODEV);	/* @q is dead */
> +		goto out_unlock;
> +	}
>  
>  	/*
>  	 * After dropping the lock and possibly sleeping here, our request
> -- 
> 1.7.3.1
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