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

[Ming Lei <ming.lei@redhat.com>]

On Wed, Jul 07, 2021 at 09:55:34AM -0400, Dan Schatzberg wrote:
> 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?
 
Good catch!

The following one line change should avoid the issue:

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 146eaa03629b..3cd51bddfec9 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -980,7 +980,6 @@ static struct loop_worker *loop_alloc_or_get_worker(struct loop_device *lo,
 
 static void loop_release_worker(struct loop_worker *worker)
 {
-	xa_erase(&worker->lo->workers, worker->blkcg_css->id);
 	css_put(worker->blkcg_css);
 	kfree(worker);
 }
@@ -1167,6 +1166,7 @@ static void __loop_free_idle_workers(struct loop_device *lo, bool force)
 		if (!force && time_is_after_jiffies(worker->last_ran_at +
 						LOOP_IDLE_WORKER_TIMEOUT))
 			break;
+		xa_erase(&worker->lo->workers, worker->blkcg_css->id);
 		if (refcount_dec_and_test(&worker->refcnt))
 			loop_release_worker(worker);
 	}


Thanks, 
Ming