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/[email protected]/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
the hardware queues are still in use by the block layer core.
Let's see what other reviewers think.
Bart.
