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From: Ming Lei <ming.lei@redhat.com>
To: Dan Schatzberg <schatzberg.dan@gmail.com>
Cc: "Jens Axboe" <axboe@kernel.dk>,
	linux-block@vger.kernel.org, "Christoph Hellwig" <hch@lst.de>,
	"Michal Koutný" <mkoutny@suse.com>
Subject: Re: [PATCH 6/6] loop: don't add worker into idle list
Date: Fri, 9 Jul 2021 08:49:55 +0800	[thread overview]
Message-ID: <YOedM4dWA3j6i4rk@T590> (raw)
In-Reply-To: <YOcWgR7Pi4+XvRZv@dschatzberg-fedora-PC0Y6AEN>

On Thu, Jul 08, 2021 at 11:15:13AM -0400, Dan Schatzberg wrote:
> On Thu, Jul 08, 2021 at 11:01:54PM +0800, Ming Lei wrote:
> > On Thu, Jul 08, 2021 at 10:16:50AM -0400, Dan Schatzberg wrote:
> > > On Thu, Jul 08, 2021 at 02:58:36PM +0800, Ming Lei wrote:
> > > > 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);
> > > >  	}
> > > 
> > > Yeah, I think this resolves the issue. You could end up repeatedly
> > > allocating workers for the same blkcg in the event that you're keeping
> > > the worker busy for the entire LOOP_IDLE_WORKER_TIMEOUT (since it only
> > > updates the last_ran_at when idle). You may want to add a racy check
> > > if the refcount is > 1 to avoid that.
> > 
> > Given the event is very unlikely to trigger, I think we can live
> > with that.
> 
> It doesn't seem unlikely to me - any workload that saturates the
> backing device would keep the loop worker constantly with at least one
> loop_cmd queued and trigger a free and allocate every
> LOOP_IDLE_WORKER_TIMEOUT. Another way to solve this is to just update
> last_ran_at before or after each loop_cmd. In any event, I'll defer to
> your decision, it's not a critical difference.

Sorry, I missed that ->last_ran_at is only set when the work isn't
pending, then we can cleanup/simplify the reclaim a bit by:

1) keep lo->idle_work to be scheduled in 60 period if there is any
active worker allocated, which is scheduled when allocating/reclaiming
one worker

2) always set ->last_ran_at after retrieving the worker from xarray,
which can be done lockless via WRITE_ONCE(), and it is cheap

3) inside __loop_free_idle_workers(), reclaim one worker only if the
worker is expired and hasn't commands in worker->cmd_list

> 
> > 
> > > 
> > > I think there might be a separate issue with the locking here though -
> > > you acquire the lo->lo_work_lock in __loop_free_idle_workers and then
> > > check worker->last_ran_at for each worker. However you only protect
> > > the write to worker->last_ran_at (in loop_process_work) with the
> > > worker->lock which I think means there's a potential data race on
> > > worker->last_ran_at.
> > 
> > It should be fine since both WRITE and READ on worker->last_ran_at is
> > atomic. Even though the race is triggered, we still can live with that.
> 
> True, though in this case I think last_ran_at should be atomic_t with
> atomic_set and atomic_read.

I think READ_ONCE()/WRITE_ONCE() should be enough, and we can set/get
last_ran_at lockless.

> 
> > 
> > 
> > On Thu, Jul 8, 2021 at 10:41 PM Dan Schatzberg <schatzberg.dan@gmail.com> wrote:
> > >
> > > On Thu, Jul 08, 2021 at 02:58:36PM +0800, Ming Lei wrote:
> > ...
> > > Another thought - do you need to change the kfree here to kfree_rcu?
> > > I'm concerned about the scenario where loop_queue_work's xa_load finds
> > > the worker and subsequently __loop_free_idle_workers erases and calls
> > > loop_release_worker. If the worker is freed then the subsequent
> > > refcount_inc_not_zero in loop_queue_work would be a use after free.
> > 
> > Good catch, will fix it in next version.
> 
> Thanks, you can go ahead and add my Acked-by to the updated version of
> this patch as well.

Thanks for the review!

-- 
Ming


  reply	other threads:[~2021-07-09  0:50 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-05 10:26 [PATCH 0/6] loop: cleanup charging io to mem/blkcg Ming Lei
2021-07-05 10:26 ` [PATCH 1/6] loop: clean up blkcg association Ming Lei
2021-07-06  5:51   ` Christoph Hellwig
2021-07-08  7:20     ` Ming Lei
2021-07-05 10:26 ` [PATCH 2/6] loop: conver timer for monitoring idle worker into dwork Ming Lei
2021-07-06  5:52   ` Christoph Hellwig
2021-07-08  7:23     ` Ming Lei
2021-07-05 10:26 ` [PATCH 3/6] loop: add __loop_free_idle_workers() for covering freeing workers in clearing FD Ming Lei
2021-07-05 10:26 ` [PATCH 4/6] loop: improve loop_process_work Ming Lei
2021-07-06  5:54   ` Christoph Hellwig
2021-07-05 10:26 ` [PATCH 5/6] loop: use xarray to store workers Ming Lei
2021-07-05 10:26 ` [PATCH 6/6] loop: don't add worker into idle list Ming Lei
2021-07-06 13:55   ` Dan Schatzberg
2021-07-07  3:19     ` Ming Lei
2021-07-07 13:55       ` Dan Schatzberg
2021-07-08  6:58         ` Ming Lei
2021-07-08 14:16           ` Dan Schatzberg
2021-07-08 15:01             ` Ming Lei
2021-07-08 15:15               ` Dan Schatzberg
2021-07-09  0:49                 ` Ming Lei [this message]
2021-07-09 13:47                   ` Dan Schatzberg
2021-07-08 14:41           ` Dan Schatzberg

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