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Subject[PATCH 1/8] blk-mq: get rid of the synchronize_rcu in __blk_mq_update_nr_hw_queues
Date
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@oracle.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
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