Re: [PATCH] blk-mq: Allow blocking queue tag iter callbacks

[Bart Van Assche <bvanassche@acm.org>]

On Tue, 2018-09-25 at 08:39 -0600, Keith Busch wrote:
> On Tue, Sep 25, 2018 at 10:39:46AM +0800, jianchao.wang wrote:
> > But the issue is the left part of blk_mq_timeout_work is moved out of protection of q refcount.
> 
> I'm not sure what you mean by "left part". The only part that isn't
> outside the reference with this patch is the part Bart pointed out.
> 
> This looks like it may be fixed by either moving the refcount back up a
> level to all the callers of blk_mq_queue_tag_busy_iter, or add
> cancel_work_sync(&q->timeout_work) to __blk_mq_update_nr_hw_queues after
> the freeze.

Hi Keith,

How about applying the following (untested) patch on top of your patch?

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 019f9b169887..099e203b5213 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -851,6 +851,9 @@ static void blk_mq_timeout_work(struct work_struct *work)
 	if (!blk_mq_queue_tag_busy_iter(q, blk_mq_check_expired, &next))
 		return;
 
+	if (!percpu_ref_tryget(&q->q_usage_counter))
+		return;
+
 	if (next != 0) {
 		mod_timer(&q->timeout, next);
 	} else {
@@ -866,6 +869,7 @@ static void blk_mq_timeout_work(struct work_struct *work)
 				blk_mq_tag_idle(hctx);
 		}
 	}
+	blk_queue_exit(q);
 }

Thanks,

Bart.