blk-mq: sync the update nr_hw_queues with blk_mq_queue_tag_busy_iter (f5bbbbe4) · Commits · 戴 / test

For blk-mq, part_in_flight/rw will invoke blk_mq_in_flight/rw to account the inflight requests. It will access the queue_hw_ctx and nr_hw_queues w/o any protection. When updating nr_hw_queues and blk_mq_in_flight/rw occur concurrently, panic comes up. Before update nr_hw_queues, the q will be frozen. So we could use q_usage_counter to avoid the race. percpu_ref_is_zero is used here so that we will not miss any in-flight request. The access to nr_hw_queues and queue_hw_ctx in blk_mq_queue_tag_busy_iter are under rcu critical section, __blk_mq_update_nr_hw_queues could use synchronize_rcu to ensure the zeroed q_usage_counter to be globally visible. Signed-off-by:

Jianchao Wang <jianchao.w.wang@oracle.com> Reviewed-by:

Ming Lei <ming.lei@redhat.com> Signed-off-by:

Jens Axboe <axboe@kernel.dk>

block/blk-mq-tag.c

+13 −1

Original line number	Diff line number	Diff line
		@@ -320,6 +320,18 @@ void blk_mq_queue_tag_busy_iter(struct request_queue q, busy_iter_fn fn,
		struct blk_mq_hw_ctx *hctx;
		int i;

		/*
		* __blk_mq_update_nr_hw_queues will update the nr_hw_queues and
		* queue_hw_ctx after freeze the queue. So we could use q_usage_counter
		* to avoid race with it. __blk_mq_update_nr_hw_queues will users
		* synchronize_rcu to ensure all of the users go out of the critical
		* section below and see zeroed q_usage_counter.
		*/
		rcu_read_lock();
		if (percpu_ref_is_zero(&q->q_usage_counter)) {
		rcu_read_unlock();
		return;
		}

		queue_for_each_hw_ctx(q, hctx, i) {
		struct blk_mq_tags *tags = hctx->tags;
		@@ -335,7 +347,7 @@ void blk_mq_queue_tag_busy_iter(struct request_queue q, busy_iter_fn fn,
		bt_for_each(hctx, &tags->breserved_tags, fn, priv, true);
		bt_for_each(hctx, &tags->bitmap_tags, fn, priv, false);
		}

		rcu_read_unlock();
		}

		static int bt_alloc(struct sbitmap_queue *bt, unsigned int depth,

block/blk-mq.c

+4 −0

Original line number	Diff line number	Diff line
		@@ -2976,6 +2976,10 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,

		list_for_each_entry(q, &set->tag_list, tag_set_list)
		blk_mq_freeze_queue(q);
		/*
		* Sync with blk_mq_queue_tag_busy_iter.
		*/
		synchronize_rcu();
		/*
		* Switch IO scheduler to 'none', cleaning up the data associated
		* with the previous scheduler. We will switch back once we are done

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