Re: [PATCH 6/6] loop: don't add worker into idle list

[Dan Schatzberg <schatzberg.dan@gmail.com>]

On Wed, Jul 07, 2021 at 11:19:14AM +0800, Ming Lei wrote:
> On Tue, Jul 06, 2021 at 09:55:36AM -0400, Dan Schatzberg wrote:
> > On Mon, Jul 05, 2021 at 06:26:07PM +0800, Ming Lei wrote:
> > >  	}
> > > +
> > > +	spin_lock(lock);
> > >  	list_add_tail(&cmd->list_entry, cmd_list);
> > > +	spin_unlock(lock);
> > >  	queue_work(lo->workqueue, work);
> > > -	spin_unlock(&lo->lo_work_lock);
> > >  }
> > >  
> > >  static void loop_update_rotational(struct loop_device *lo)
> > > @@ -1131,20 +1159,18 @@ static void loop_set_timer(struct loop_device *lo)
> > >  
> > >  static void __loop_free_idle_workers(struct loop_device *lo, bool force)
> > >  {
> > > -	struct loop_worker *pos, *worker;
> > > +	struct loop_worker *worker;
> > > +	unsigned long id;
> > >  
> > >  	spin_lock(&lo->lo_work_lock);
> > > -	list_for_each_entry_safe(worker, pos, &lo->idle_worker_list,
> > > -				idle_list) {
> > > +	xa_for_each(&lo->workers, id, worker) {
> > >  		if (!force && time_is_after_jiffies(worker->last_ran_at +
> > >  						LOOP_IDLE_WORKER_TIMEOUT))
> > >  			break;
> > > -		list_del(&worker->idle_list);
> > > -		xa_erase(&lo->workers, worker->blkcg_css->id);
> > > -		css_put(worker->blkcg_css);
> > > -		kfree(worker);
> > > +		if (refcount_dec_and_test(&worker->refcnt))
> > > +			loop_release_worker(worker);
> > 
> > This one is puzzling to me. Can't you hit this refcount decrement
> > superfluously each time the loop timer fires?
> 
> Not sure I get your point.
> 
> As I mentioned above, this one is the counter pair of INIT reference,
> but one new lo_cmd may just grab it when queueing rq before erasing the
> worker from xarray, so we can't release worker here until the command is
> completed.

Suppose at this point there's still an outstanding loop_cmd to be
serviced for this worker. The refcount_dec_and_test should decrement
the refcount and then fail the conditional, not calling
loop_release_worker. What happens if __loop_free_idle_workers fires
again before the loop_cmd is processed? Won't you decrement the
refcount again, and then end up calling loop_release_worker before the
loop_cmd is processed?