linux-kernel - [PATCH 1/8] blk-mq: get rid of the synchronize_rcu in __blk_mq_update_nr_hw

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Date:   Fri, 15 Mar 2019 16:57:37 +0800
From:   Jianchao Wang <jianchao.w.wang@...cle.com>
To:     axboe@...nel.dk
Cc:     hch@....de, jthumshirn@...e.de, hare@...e.de, josef@...icpanda.com,
        bvanassche@....org, sagi@...mberg.me, keith.busch@...el.com,
        jsmart2021@...il.com, linux-block@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: [PATCH 1/8] blk-mq: get rid of the synchronize_rcu in __blk_mq_update_nr_hw_queues

In commit 530ca2c (blk-mq: Allow blocking queue tag iter callbacks),
we try to get a non-zero q_usage_counter to avoid access hctxs that
being modified. So the synchronize_rcu is useless and should be
removed.

Signed-off-by: Jianchao Wang <jianchao.w.wang@...cle.com>
---
 block/blk-mq-tag.c | 4 +---
 block/blk-mq.c     | 4 ----
 2 files changed, 1 insertion(+), 7 deletions(-)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 2089c6c..b792537 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -384,9 +384,7 @@ void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
 	/*
 	 * __blk_mq_update_nr_hw_queues() updates nr_hw_queues and queue_hw_ctx
 	 * while the queue is frozen. So we can use q_usage_counter to avoid
-	 * racing with it. __blk_mq_update_nr_hw_queues() uses
-	 * synchronize_rcu() to ensure this function left the critical section
-	 * below.
+	 * racing with it.
 	 */
 	if (!percpu_ref_tryget(&q->q_usage_counter))
 		return;
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 9437a5e..07584a9 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -3208,10 +3208,6 @@ 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
 	 * updating the new sw to hw queue mappings.
-- 
2.7.4