Re: [PATCH v2 1/2] blk-mq: add tagset quiesce interface

[Christoph Hellwig <hch@lst.de>]

On Thu, Oct 13, 2022 at 05:44:49PM +0800, Chao Leng wrote:
> +	rcu = kvmalloc(count * sizeof(*rcu), GFP_KERNEL);
> +	if (rcu) {
> +		list_for_each_entry(q, &set->tag_list, tag_set_list) {
> +			if (blk_queue_noquiesced(q))
> +				continue;
> +
> +			init_rcu_head(&rcu[i].head);
> +			init_completion(&rcu[i].completion);
> +			call_srcu(q->srcu, &rcu[i].head, wakeme_after_rcu);
> +			i++;
> +		}
> +
> +		for (i = 0; i < count; i++) {
> +			wait_for_completion(&rcu[i].completion);
> +			destroy_rcu_head(&rcu[i].head);
> +		}
> +		kvfree(rcu);
> +	} else {
> +		list_for_each_entry(q, &set->tag_list, tag_set_list)
> +			synchronize_srcu(q->srcu);
> +	}

Having to allocate a struct rcu_synchronize for each of the potentially
many queues here is a bit sad.

Pull just explained the start_poll_synchronize_rcu interfaces at ALPSS
last week, so I wonder if something like that would also be feasible
for SRCU, as that would come in really handy here.