blk-mq: Allow blocking queue tag iter callbacks

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@oracle.com>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 4 insertions, 9 deletions

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 94e1ed667b6e..41317c50a446 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -322,16 +322,11 @@ void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
 
         /*
          * __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.
+         * queue_hw_ctx after freeze the queue, so we use q_usage_counter
+         * to avoid race with it.
          */
-        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;
@@ -347,7 +342,7 @@ 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();
+        blk_queue_exit(q);
 }
 
 static int bt_alloc(struct sbitmap_queue *bt, unsigned int depth,