Re: [RFC PATCH] blk-mq: implement queue quiesce via percpu_ref for BLK_MQ_F_BLOCKING

Keith Busch
2020-07-29T16:12:33Z

On Tue, Jul 28, 2020 at 09:49:38PM +0800, Ming Lei wrote:
> void blk_mq_quiesce_queue(struct request_queue *q)
> {
> - struct blk_mq_hw_ctx *hctx;
> - unsigned int i;
> - bool rcu = false;
> -
> blk_mq_quiesce_queue_nowait(q);
>
> - queue_for_each_hw_ctx(q, hctx, i) {
> - if (hctx->flags & BLK_MQ_F_BLOCKING)
> - synchronize_srcu(hctx->srcu);
> - else
> - rcu = true;
> - }
> - if (rcu)
> + if (q->tag_set->flags & BLK_MQ_F_BLOCKING) {
> + percpu_ref_kill(&q->dispatch_counter);
> + wait_event(q->mq_quiesce_wq,
> + percpu_ref_is_zero(&q->dispatch_counter));
> + } else
> synchronize_rcu();
> }

> +static void hctx_lock(struct blk_mq_hw_ctx *hctx)
> {
> - if (!(hctx->flags & BLK_MQ_F_BLOCKING)) {
> - /* shut up gcc false positive */
> - *srcu_idx = 0;
> + if (!(hctx->flags & BLK_MQ_F_BLOCKING))
> rcu_read_lock();
> - } else
> - *srcu_idx = srcu_read_lock(hctx->srcu);
> + else
> + percpu_ref_get(&hctx->queue->dispatch_counter);
> }

percpu_ref_get() will always succeed, even after quiesce kills it.
Isn't it possible that 'percpu_ref_is_zero(&q->dispatch_counter))' may
never reach 0? We only need to ensure that dispatchers will observe
blk_queue_quiesced(). That doesn't require that there are no current
dispatchers.