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

[Paul E. McKenney]

On Fri, Mar 24, 2023 at 12:55:23AM +0000, Joel Fernandes wrote:
> On Wed, Mar 22, 2023 at 04:18:24PM -0700, Paul E. McKenney wrote:
> > On Wed, Mar 22, 2023 at 08:44:53PM +0100, Frederic Weisbecker wrote:
> > > The shrinker may run concurrently with callbacks (de-)offloading. As
> > > such, calling rcu_nocb_lock() is very dangerous because it does a
> > > conditional locking. The worst outcome is that rcu_nocb_lock() doesn't
> > > lock but rcu_nocb_unlock() eventually unlocks, or the reverse, creating
> > > an imbalance.
> > > 
> > > Fix this with protecting against (de-)offloading using the barrier mutex.
> > > 
> > > Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> > 
> > Good catch!!!  A few questions, comments, and speculations below.
> 
> Added a few more. ;)
> 
> > > ---
> > >  kernel/rcu/tree_nocb.h | 17 ++++++++++++++++-
> > >  1 file changed, 16 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> > > index f2280616f9d5..dd9b655ae533 100644
> > > --- a/kernel/rcu/tree_nocb.h
> > > +++ b/kernel/rcu/tree_nocb.h
> > > @@ -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?
> 
> Maybe trylock is better then? If we can't make progress, may be better to let
> kswapd free memory by other means than blocking on the mutex.
> 
> ISTR, from my Android days that there are weird lockdep issues that happen
> when locking in a shrinker (due to the 'fake lock' dependency added during
> reclaim).

This stuff gets tricky quickly.  ;-)

> > The other concern was that the (de-)offloading operation might take a
> > long time, but the usual cause for that is huge numbers of callbacks,
> > in which case letting them free their memory is not necessarily a bad
> > strategy.
> > 
> > > +
> > >  	/* 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?
> 
> Did you mean de-offloaded? If it is offloaded, that means nocb is active so
> there could be lazy CBs queued. Or did I miss something?

You are quite right, offloaded for ->lazy_len to be zero.

							Thanx, Paul.

> thanks,
> 
>  - Joel
> 
> 
> > Or can it contain garbage after a de-offloading operation?
> > 
> > > +		_count = READ_ONCE(rdp->lazy_len);
> > >  
> > >  		if (_count == 0)
> > >  			continue;
> > > +
> > >  		rcu_nocb_lock_irqsave(rdp, flags);
> > >  		WRITE_ONCE(rdp->lazy_len, 0);
> > >  		rcu_nocb_unlock_irqrestore(rdp, flags);
> > > @@ -1352,6 +1364,9 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
> > >  		if (sc->nr_to_scan <= 0)
> > >  			break;
> > >  	}
> > > +
> > > +	mutex_unlock(&rcu_state.barrier_mutex);
> > > +
> > >  	return count ? count : SHRINK_STOP;
> > >  }
> > >  
> > > -- 
> > > 2.34.1
> > >