On 4/1/19 10:39 AM, Bart Van Assche wrote: > On 3/31/19 7:00 PM, Ming Lei wrote: >> On Sun, Mar 31, 2019 at 08:27:35AM -0700, Bart Van Assche wrote: >>> I'm not sure the approach of this patch series is really the direction we >>> should pursue. There are many block driver that free resources immediately >> >> Please see scsi_run_queue(), and the queue refcount is always held >> before run queue. > > That's not correct. There is no guarantee that q->q_usage_counter > 0 when scsi_run_queue() is called from inside scsi_requeue_run_queue(). > >>> I'd like to avoid having to modify all block drivers that free resources >>> immediately after blk_cleanup_queue() has returned. Have you considered to >>> modify blk_mq_run_hw_queues() such that it becomes safe to call that >>> function while blk_cleanup_queue() is in progress, e.g. by inserting a >>> percpu_ref_tryget_live(&q->q_usage_counter) / >>> percpu_ref_put(&q->q_usage_counter) pair? >> >> It can't work because blk_mq_run_hw_queues may happen after >> percpu_ref_exit() is done. >> >> However, if we move percpu_ref_exit() into queue's release handler, we >> don't need to grab q->q_usage_counter any more in blk_mq_run_hw_queues(), >> and we still have to free hw queue resources in queue's release handler, >> that is exactly what this patchset is doing. >> >> In short, getting q->q_usage_counter doesn't make a difference on this >> issue. > > percpu_ref_tryget_live() fails if a per-cpu counter is in the "dead" state. percpu_ref_kill() changes the state of a per-cpu counter to the "dead" state. blk_freeze_queue_start() calls percpu_ref_kill(). blk_cleanup_queue() already calls blk_set_queue_dying() and that last function calls blk_freeze_queue_start(). So I think that what you wrote is not correct and that inserting a percpu_ref_tryget_live()/percpu_ref_put() pair in blk_mq_run_hw_queues() or blk_mq_run_hw_queue() would make a difference and also that moving the percpu_ref_exit() call into blk_release_queue() makes sense. > percpu_ref_tryget would be better to get pending requests to be issued when queue is frozen. Thanks Jianchao
