On 4/1/19 11:27 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(). >> >>>> 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. > > This way is easy to cause deadlock!!! > > If percpu_ref_tryget_live() is called in the entry of blk_mq_run_hw_queues(), > at the same time, blk_freeze_queue_start() is called, then percpu_ref_tryget_live() > will fail, and run queue can't move on, then blk_mq_freeze_queue_wait() will wait > forever. > percpu_ref_tryget would be better to get pending requests to be issued when queue is frozen. Thanks Jianchao
