[PATCH v2 3/4] blk-mq: sync the update nr_hw_queues with blk_mq_queue_tag_busy_iter

[Giuliano Procida <gprocida@google.com>]

commit f5bbbbe4d63577026f908a809f22f5fd5a90ea1f upstream.

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.

NOTE: Back-ported to 4.4.y.

The upstream commit was intended to prevent concurrent manipulation of
nr_hw_queues and iteration over queues. The former doesn't happen in
this in 4.4.7 (as __blk_mq_update_nr_hw_queues doesn't exist). The
extra locking is also buggy in this commit but fixed in a follow-up.

It may protect against other concurrent accesses such as queue removal
by synchronising RCU locking around q_usage_counter.

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>
Signed-off-by: Giuliano Procida <gprocida@google.com>
---
 block/blk-mq-tag.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index a07ca3488d96..bf356de30134 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -481,6 +481,14 @@ void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
 	struct blk_mq_hw_ctx *hctx;
 	int i;
 
+	/*
+	 * Avoid potential races with things like queue removal.
+	 */
+	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;
@@ -497,7 +505,7 @@ void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
 		bt_for_each(hctx, &tags->bitmap_tags, tags->nr_reserved_tags, fn, priv,
 		      false);
 	}
-
+	rcu_read_unlock();
 }
 
 static unsigned int bt_unused_tags(struct blk_mq_bitmap_tags *bt)
-- 
2.26.0.110.g2183baf09c-goog