On Fri, Aug 17, 2018 at 11:54 AM, Jianchao Wang
<jianchao.w.wang@xxxxxxxxxx> wrote:
> 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. And also both the
> check and blk_mq_queue_tag_busy_iter are under rcu critical section,
> then __blk_mq_update_nr_hw_queues could ensure the zeroed q_usage_counter
> to be globally visible.
>
> Signed-off-by: Jianchao Wang <jianchao.w.wang@xxxxxxxxxx>
> ---
> block/blk-mq.c | 31 ++++++++++++++++++++++++++++++-
> 1 file changed, 30 insertions(+), 1 deletion(-)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index de7027f..9ec98bd 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -112,7 +112,22 @@ void blk_mq_in_flight(struct request_queue *q, struct hd_struct *part,
> struct mq_inflight mi = { .part = part, .inflight = inflight, };
>
> inflight[0] = inflight[1] = 0;
> +
> + /*
> + * __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 use
> + * synchronize_rcu to ensure all of the users of blk_mq_in_flight
> + * go out of the critical section and see zeroed q_usage_counter.
> + */
> + rcu_read_lock();
> + if (percpu_ref_is_zero(&q->q_usage_counter)) {
> + rcu_read_unlock();
> + return;
> + }
> +
> blk_mq_queue_tag_busy_iter(q, blk_mq_check_inflight, &mi);
> + rcu_read_unlock();
> }
>
> static void blk_mq_check_inflight_rw(struct blk_mq_hw_ctx *hctx,
> @@ -131,7 +146,18 @@ void blk_mq_in_flight_rw(struct request_queue *q, struct hd_struct *part,
> struct mq_inflight mi = { .part = part, .inflight = inflight, };
>
> inflight[0] = inflight[1] = 0;
> +
> + /*
> + * See comment of blk_mq_in_flight.
> + */
> + rcu_read_lock();
> + if (percpu_ref_is_zero(&q->q_usage_counter)) {
> + rcu_read_unlock();
> + return;
> + }
> +
> blk_mq_queue_tag_busy_iter(q, blk_mq_check_inflight_rw, &mi);
> + rcu_read_unlock();
I'd suggest to put the rcu_* and percpu_ref_is_zero() into
blk_mq_queue_tag_busy_iter().
Thanks,
Ming Lei
