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

Frederic Weisbecker
2023-03-24T22:09:16Z

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?

> > /* 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.

Thanks.