Re: [PATCH V2] SCSI: fix queue cleanup race before queue initialization is done

[Bart Van Assche <bvanassche@acm.org>]

On Wed, 2018-11-14 at 16:25 +0800, Ming Lei wrote:
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -697,6 +697,12 @@ static bool scsi_end_request(struct request *req,
> blk_status_t error,
>  		 */
>  		scsi_mq_uninit_cmd(cmd);
>  
> +		/*
> +		 * queue is still alive, so grab the ref for preventing it
> +		 * from being cleaned up during running queue.
> +		 */
> +		percpu_ref_get(&q->q_usage_counter);
> +

I think the above comment is misleading. In the block layer a queue is called
alive if the "dying" flag has not been set. When the above call to
percpu_ref_get() occurs it is not guaranteed that that flag has not yet been
set. But it is guaranteed that q->q_usage_counter is not zero. I would prefer
if the comment would be modified.

What's not clear to me is why this patch only protects the blk-mq path but
not the legacy path. Does the legacy path need similar protection? It also
triggers a queue run after having finished a request.

Otherwise this patch looks fine to me.

Bart.