Re: [PATCH v5 1/2] blk-mq: add tagset quiesce interface

[Christoph Hellwig <hch@lst.de>]

On Tue, Jul 28, 2020 at 06:10:42PM +0800, Ming Lei wrote:
> +	struct blk_mq_hw_ctx *hctx = (void *)srcu -
> +		sizeof(struct blk_mq_hw_ctx);

I think this should use container_of.

> +{
> +	struct blk_mq_hw_ctx *hctx;
> +	unsigned int i;
> +
> +	blk_mq_quiesce_queue_nowait(q);
> +
> +	queue_for_each_hw_ctx(q, hctx, i) {
> +		WARN_ON_ONCE(!(hctx->flags & BLK_MQ_F_BLOCKING));
> +		init_rcu_head(&hctx->srcu->head);

We only need to initialize the rcu head once when we allocate the hctx.

> +	mutex_lock(&set->tag_list_lock);
> +	if (set->flags & BLK_MQ_F_BLOCKING) {
> +		int count = 0;
> +
> +		list_for_each_entry(q, &set->tag_list, tag_set_list)
> +			count++;
> +
> +		atomic_set(&set->quiesce_count, count);
> +		init_completion(&set->quiesce_completion);
> +
> +		list_for_each_entry(q, &set->tag_list, tag_set_list)
> +			blk_mq_quiesce_blocking_queue_async(q);
> +		wait_for_completion(&set->quiesce_completion);
> +	} else {
> +		list_for_each_entry(q, &set->tag_list, tag_set_list)
> +			blk_mq_quiesce_queue_nowait(q);
> +		synchronize_rcu();
> +	}
> +	mutex_unlock(&set->tag_list_lock);

It would be great to do the wait_for_completion and synchronize_rcu
outside the lock.  Also I think the blocking case probably wants to be
split into a separate little helper.