Re: [PATCH 8/9] block: allow to allocate req with REQF_PREEMPT when queue is frozen

[Bart Van Assche <Bart.VanAssche@xxxxxxx>]

On Fri, 2017-09-01 at 01:27 +0800, Ming Lei wrote:
> @@ -1413,9 +1414,17 @@ static struct request *blk_old_get_request(struct request_queue *q,
>  	/* create ioc upfront */
>  	create_io_context(gfp_mask, q->node);
>  
> -	ret = blk_queue_enter(q, !(gfp_mask & __GFP_DIRECT_RECLAIM));
> +	/*
> +	 * When queue is frozen, we still need to allocate req for
> +	 * REQF_PREEMPT.
> +	 */
> +	if ((flags & BLK_MQ_REQ_PREEMPT) && blk_queue_is_frozen(q))
> +		blk_queue_enter_live(q);
> +	else
> +		ret = blk_queue_enter(q, !(gfp_mask & __GFP_DIRECT_RECLAIM));

Hello Ming,

Sorry but I doubt that calling blk_queue_enter_live() from inside
blk_old_get_request() is safe. Calling blk_queue_enter_live() is only safe
before a request queue has been marked dead. What prevents a kernel thread
that holds a reference on a request queue and that is running concurrently
with blk_cleanup_queue() to call blk_old_get_request() after a queue has
been marked dead?

Thanks,

Bart.