[LSF/MM/BPF TOPIC] Improving Zoned Storage Support

[Bart Van Assche]

On 1/17/24 13:14, Jens Axboe wrote:
>   /* Maps an I/O priority class to a deadline scheduler priority. */
> @@ -600,6 +604,10 @@ static struct request *dd_dispatch_request(struct blk_mq_hw_ctx *hctx)
>   	struct request *rq;
>   	enum dd_prio prio;
>   
> +	if (test_bit(0, &dd->dispatch_state) ||
> +	    test_and_set_bit(0, &dd->dispatch_state))
> +		return NULL;
> +
>   	spin_lock(&dd->lock);
>   	rq = dd_dispatch_prio_aged_requests(dd, now);
>   	if (rq)
> @@ -616,6 +624,7 @@ static struct request *dd_dispatch_request(struct blk_mq_hw_ctx *hctx)
>   	}
>   
>   unlock:
> +	clear_bit(0, &dd->dispatch_state);
>   	spin_unlock(&dd->lock);

Can the above code be simplified by using spin_trylock() instead of
test_bit() and test_and_set_bit()?

Please note that whether or not spin_trylock() is used, there is a
race condition in this approach: if dd_dispatch_request() is called
just before another CPU calls spin_unlock() from inside 
dd_dispatch_request() then some requests won't be dispatched until
the next time dd_dispatch_request() is called.

Thanks,

Bart.