On Tue, Apr 02, 2019 at 08:44:10AM -0700, Bart Van Assche wrote:
> On Tue, 2019-04-02 at 08:53 +0800, Ming Lei wrote:
> > On Mon, Apr 01, 2019 at 02:20:12PM -0700, Bart Van Assche wrote:
> > > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > > index 3ff3d7b49969..652d0c6d5945 100644
> > > --- a/block/blk-mq.c
> > > +++ b/block/blk-mq.c
> > > @@ -1499,12 +1499,20 @@ void blk_mq_run_hw_queues(struct request_queue *q, bool async)
> > > struct blk_mq_hw_ctx *hctx;
> > > int i;
> > >
> > > + /*
> > > + * Do not run any hardware queues if the queue is frozen or if a
> > > + * concurrent blk_cleanup_queue() call is removing any data
> > > + * structures used by this function.
> > > + */
> > > + if (!percpu_ref_tryget(&q->q_usage_counter))
> > > + return;
> > > queue_for_each_hw_ctx(q, hctx, i) {
> > > if (blk_mq_hctx_stopped(hctx))
> > > continue;
> > >
> > > blk_mq_run_hw_queue(hctx, async);
> > > }
> > > + percpu_ref_put(&q->q_usage_counter);
> > > }
> > > EXPORT_SYMBOL(blk_mq_run_hw_queues);
> >
> > I don't see it is necessary to add percpu_ref_tryget()/percpu_ref_put()
> > in the fast path if we simply release all hctx resource in hctx's
> > release handler by the following patch:
> >
> > https://lore.kernel.org/linux-block/20190401044247.29881-2-ming.lei@xxxxxxxxxx/T/#u
> >
> > Even we can kill the percpu_ref_tryget_live()/percpu_ref_put() in
> > scsi_end_request().
>
> The above approach has the advantages of being easy to review and to maintain.
>
> Patch "[PATCH V2 1/3] blk-mq: free hw queue's resource in hctx's release handler"
> makes the block layer more complicated because it introduces a new state for
> hardware queues: block driver cleanup has happened (set->ops->exit_hctx(...)) but
We are done with driver after blk_freeze_queue() and blk_sync_queue(),
then call .exit_hctx() to say good bye with driver, I don't see it
causes any issue.
> the hardware queues are still in use by the block layer core.
Block layer has the correct in-memory state to work well, and no
driver activity is involved too.
Thanks,
Ming
