Hi Dongli, On Mon, Apr 01, 2019 at 01:05:46PM +0800, Dongli Zhang wrote: > > > On 4/1/19 10:52 AM, Ming Lei wrote: > > On Sun, Mar 31, 2019 at 07:39:17PM -0700, 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(). > > > > We don't need the guarantee of 'q->q_usage_counter > 0', I mean the > > queue's kobj reference counter. > > > > What we need is to allow run queue to work correctly after queue is frozen > > or cleaned up. > > > >> > >>>> 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. > > > > If percpu_ref_exit() is moved to blk_release_queue(), we still need to > > move freeing of hw queue's resource into blk_release_queue() like what > > the patchset is doing. > > Hi Ming, > > Would you mind help explain why we still need to move freeing of hw queue's > resource into blk_release_queue() like what the patchset is doing? > > Let's assume there is no deadlock when percpu_ref_tryget_live() is used, Could you explain why the assumption is true? We have to run queue after starting to freeze queue for draining allocated requests and making forward progress. Inside blk_freeze_queue_start(), percpu_ref_kill() marks this ref as DEAD, then percpu_ref_tryget_live() returns false, then queue won't be run. Thanks, Ming
