Re: [PATCH 1/4] rcu/nocb: Protect lazy shrinker against concurrent (de-)offloading

[Frederic Weisbecker]

Le Fri, Mar 24, 2023 at 03:51:54PM -0700, Paul E. McKenney a écrit :
> On Fri, Mar 24, 2023 at 11:09:08PM +0100, Frederic Weisbecker wrote:
> > Le Wed, Mar 22, 2023 at 04:18:24PM -0700, Paul E. McKenney a écrit :
> > > > @@ -1336,13 +1336,25 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
> > > >  	unsigned long flags;
> > > >  	unsigned long count = 0;
> > > >  
> > > > +	/*
> > > > +	 * Protect against concurrent (de-)offloading. Otherwise nocb locking
> > > > +	 * may be ignored or imbalanced.
> > > > +	 */
> > > > +	mutex_lock(&rcu_state.barrier_mutex);
> > > 
> > > I was worried about this possibly leading to out-of-memory deadlock,
> > > but if I recall correctly, the (de-)offloading process never allocates
> > > memory, so this should be OK?
> > 
> > Good point. It _should_ be fine but like you, Joel and Hillf pointed out
> > it's asking for trouble.
> > 
> > We could try Joel's idea to use mutex_trylock() as a best effort, which
> > should be fine as it's mostly uncontended.
> > 
> > The alternative is to force nocb locking and check the offloading state
> > right after. So instead of:
> > 
> > 	rcu_nocb_lock_irqsave(rdp, flags);
> > 	//flush stuff
> > 	rcu_nocb_unlock_irqrestore(rdp, flags);
> > 
> > Have:
> > 
> > 	raw_spin_lock_irqsave(rdp->nocb_lock, flags);
> > 	if (!rcu_rdp_is_offloaded(rdp))
> > 		raw_spin_unlock_irqrestore(rdp->nocb_lock, flags);
> > 		continue;
> > 	}
> > 	//flush stuff
> > 	rcu_nocb_unlock_irqrestore(rdp, flags);
> > 
> > But it's not pretty and also disqualifies the last two patches as
> > rcu_nocb_mask can't be iterated safely anymore.
> > 
> > What do you think?
> 
> The mutex_trylock() approach does have the advantage of simplicity,
> and as you say should do well given low contention.
> 
> Which reminds me, what sort of test strategy did you have in mind?
> Memory exhaustion can have surprising effects.

The best I can do is to trigger the count and scan callbacks through
the shrinker debugfs and see if it crashes or not :-)

> 
> > > >  	/* Snapshot count of all CPUs */
> > > >  	for_each_possible_cpu(cpu) {
> > > >  		struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
> > > > -		int _count = READ_ONCE(rdp->lazy_len);
> > > > +		int _count;
> > > > +
> > > > +		if (!rcu_rdp_is_offloaded(rdp))
> > > > +			continue;
> > > 
> > > If the CPU is offloaded, isn't ->lazy_len guaranteed to be zero?
> > > 
> > > Or can it contain garbage after a de-offloading operation?
> > 
> > If it's deoffloaded, ->lazy_len is indeed (supposed to be) guaranteed to be zero.
> > Bypass is flushed and disabled atomically early on de-offloading and the
> > flush resets ->lazy_len.
> 
> Whew!  At the moment, I don't feel strongly about whether or not
> the following code should (1) read the value, (2) warn on non-zero,
> (3) assume zero without reading, or (4) some other option that is not
> occurring to me.  Your choice!

(2) looks like a good idea!

Thanks.