[PATCH 4.4 66/86] blk-mq: Allow blocking queue tag iter callbacks

[Greg Kroah-Hartman]

From: Keith Busch <keith.busch@xxxxxxxxx>

commit 530ca2c9bd6949c72c9b5cfc330cb3dbccaa3f5b upstream.

A recent commit runs tag iterator callbacks under the rcu read lock,
but existing callbacks do not satisfy the non-blocking requirement.
The commit intended to prevent an iterator from accessing a queue that's
being modified. This patch fixes the original issue by taking a queue
reference instead of reading it, which allows callbacks to make blocking
calls.

Fixes: f5bbbbe4d6357 ("blk-mq: sync the update nr_hw_queues with blk_mq_queue_tag_busy_iter")
Acked-by: Jianchao Wang <jianchao.w.wang@xxxxxxxxxx>
Signed-off-by: Keith Busch <keith.busch@xxxxxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Giuliano Procida <gprocida@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 block/blk-mq-tag.c |    7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -484,11 +484,8 @@ void blk_mq_queue_tag_busy_iter(struct r
 	/*
 	 * Avoid potential races with things like queue removal.
 	 */
-	rcu_read_lock();
-	if (percpu_ref_is_zero(&q->q_usage_counter)) {
-		rcu_read_unlock();
+	if (!percpu_ref_tryget(&q->q_usage_counter))
 		return;
-	}
 
 	queue_for_each_hw_ctx(q, hctx, i) {
 		struct blk_mq_tags *tags = hctx->tags;
@@ -505,7 +502,7 @@ void blk_mq_queue_tag_busy_iter(struct r
 		bt_for_each(hctx, &tags->bitmap_tags, tags->nr_reserved_tags, fn, priv,
 		      false);
 	}
-	rcu_read_unlock();
+	blk_queue_exit(q);
 }
 
 static unsigned int bt_unused_tags(struct blk_mq_bitmap_tags *bt)