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    SubjectRe: [PATCH 2/2] blk-mq: sync the update nr_hw_queues with part_in_flight
    On Thu, Aug 16, 2018 at 05:20:50PM +0800, jianchao.wang wrote:
    > Hi Ming
    >
    > On 08/16/2018 05:03 PM, Ming Lei wrote:
    > > diff --git a/block/blk-mq.c b/block/blk-mq.c
    > > index b42a2c9ba00e..fbc5534f8178 100644
    > > --- a/block/blk-mq.c
    > > +++ b/block/blk-mq.c
    > > @@ -113,6 +113,10 @@ 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;
    > > +
    > > + if (percpu_ref_is_dying(&q->q_usage_counter))
    > > + return;
    > > +
    > > blk_mq_queue_tag_busy_iter(q, blk_mq_check_inflight, &mi);
    > > }
    >
    > That's a good idea to use q->q_usage_counter.
    > But I think we could do following modification:
    > 1. use percpu_ref_is_zero, then we will not miss any in-flight request here.
    > 2. use rcu to ensure the user of blk_mq_in_flight has gone out of the critical section.
    > Like following patch:
    > diff --git a/block/blk-mq.c b/block/blk-mq.c
    > index 89904cc..cd9878e 100644
    > --- a/block/blk-mq.c
    > +++ b/block/blk-mq.c
    > @@ -113,7 +113,12 @@ void blk_mq_in_flight(struct request_queue *q, struct hd_struct *part,
    >
    > inflight[0] = inflight[1] = 0;
    >
    > + rcu_read_lock();
    > + if (percpu_ref_is_zero(&q->q_usage_counter))
    > + 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,
    > @@ -2907,6 +2912,7 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
    > list_for_each_entry(q, &set->tag_list, tag_set_list)
    > blk_mq_freeze_queue(q);
    >
    > + synchronize_rcu();
    > /*
    > * switch io scheduler to NULL to clean up the data in it.
    > * will get it back after update mapping between cpu and hw queues.
    >
    > And also, some comment is needed to describe them. ;)

    This patch looks fine for me.

    Thanks
    Ming

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