On Mon, 2017-03-27 at 20:06 +0800, Ming Lei wrote:
> Before commit 780db2071a(blk-mq: decouble blk-mq freezing
> from generic bypassing), the dying flag is checked before
> entering queue, and Tejun converts the checking into .mq_freeze_depth,
> and assumes the counter is increased just after dying flag
> is set. Unfortunately we doesn't do that in blk_set_queue_dying().
>
> This patch calls blk_freeze_queue_start() in blk_set_queue_dying(),
> so that we can block new I/O coming once the queue is set as dying.
>
> Given blk_set_queue_dying() is always called in remove path
> of block device, and queue will be cleaned up later, we don't
> need to worry about undoing the counter.
>
> Cc: Bart Van Assche <bart.vanassche@xxxxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Reviewed-by: Hannes Reinecke <hare@xxxxxxxx>
> Signed-off-by: Ming Lei <tom.leiming@xxxxxxxxx>
> ---
> block/blk-core.c | 13 ++++++++++---
> 1 file changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 60f364e1d36b..e22c4ea002ec 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -500,6 +500,13 @@ void blk_set_queue_dying(struct request_queue *q)
> queue_flag_set(QUEUE_FLAG_DYING, q);
> spin_unlock_irq(q->queue_lock);
>
> + /*
> + * When queue DYING flag is set, we need to block new req
> + * entering queue, so we call blk_freeze_queue_start() to
> + * prevent I/O from crossing blk_queue_enter().
> + */
> + blk_freeze_queue_start(q);
>
> if (q->mq_ops)
> blk_mq_wake_waiters(q);
> else {
> @@ -672,9 +679,9 @@ int blk_queue_enter(struct request_queue *q, bool nowait)
> /*
> * read pair of barrier in blk_freeze_queue_start(),
> * we need to order reading __PERCPU_REF_DEAD flag of
> - * .q_usage_counter and reading .mq_freeze_depth,
> - * otherwise the following wait may never return if the
> - * two reads are reordered.
> + * .q_usage_counter and reading .mq_freeze_depth or
> + * queue dying flag, otherwise the following wait may
> + * never return if the two reads are reordered.
> */
> smp_rmb();
>
An explanation of why that crossing can happen is still missing above the
blk_freeze_queue_start() call. Additionally, I'm still wondering whether
or not we need "Cc: stable" tags for the patches in this series. But since
the code looks fine:
Reviewed-by: Bart Van Assche <bart.vanassche@xxxxxxxxxxx>
>
