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)