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

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Date:   Wed, 29 Mar 2023 23:18:24 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, rcu <rcu@...r.kernel.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH 1/4] rcu/nocb: Protect lazy shrinker against concurrent
 (de-)offloading

On Wed, Mar 29, 2023 at 01:44:53PM -0700, Paul E. McKenney wrote:
> On Wed, Mar 29, 2023 at 06:02:00PM +0200, Frederic Weisbecker wrote:
> > +		/*
> > +		 * But really don't insist if barrier_mutex is contended since we
> > +		 * can't guarantee that it will never engage in a dependency
> > +		 * chain involving memory allocation. The lock is seldom contended
> > +		 * anyway.
> > +		 */
> > +		return 0;
> > +	}
> > +
> >  	/* 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;
> > +
> > +		_count = READ_ONCE(rdp->lazy_len);
> >  
> >  		if (_count == 0)
> >  			continue;
> > +
> 
> And I just might have unconfused myself here.  We get here only if this
> CPU is offloaded, in which case it might also have non-zero ->lazy_len,
> so this is in fact *not* dead code.

Right. Now whether it's really alive remains to be proven ;)