linux-kernel - Re: Question on comment header for for_each

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Message-Id: <20181108161122.GJ4170@linux.ibm.com>
Date:   Thu, 8 Nov 2018 08:11:22 -0800
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: Question on comment header for for_each_domain()

On Thu, Nov 08, 2018 at 04:35:40PM +0100, Peter Zijlstra wrote:
> On Thu, Nov 08, 2018 at 07:31:09AM -0800, Paul E. McKenney wrote:
> > On Thu, Nov 08, 2018 at 10:21:51AM +0100, Peter Zijlstra wrote:
> > > On Wed, Nov 07, 2018 at 03:00:02PM -0800, Paul E. McKenney wrote:
> > > > Hello!
> > > > 
> > > > The header comment for for_each_domain() talks about a call to
> > > > synchronize_sched() within detach_destroy_domains(), but I am not
> > > > seeing any such call.  Because synchronize_sched() is now folded into
> > > > synchronize_rcu(), I have a patch that edits the comment, but it looks
> > > > like a larger change is needed.
> > > > 
> > > > Or am I blind today?
> > > 
> > > I think you're quite right and that comment is a wee bit stale.
> > > 
> > > The sched domain tree is indeed protected by regular RCU (not RCU-sched
> > > as the comment seems to imply) and this is per destroy_sched_domains()
> > > using call_rcu().
> > > 
> > > And most (I didn't look at all) uses for the sched-domain tree do indeed
> > > employ rcu_read_lock().
> > 
> > Ah, thank you for the info!  Would this patch do the trick?
> 
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index 618577fc9aa8..00b91d16af9f 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -1237,7 +1237,7 @@ extern void sched_ttwu_pending(void);
> >  
> >  /*
> >   * The domain tree (rq->sd) is protected by RCU's quiescent state transition.
> > - * See detach_destroy_domains: synchronize_sched for details.
> > + * See destroy_sched_domains: call_rcu for details.
> >   *
> >   * The domain tree of any CPU may only be accessed from within
> >   * preempt-disabled sections.
> > diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> > index 8d7f15ba5916..04d458faf2c1 100644
> > --- a/kernel/sched/topology.c
> > +++ b/kernel/sched/topology.c
> > @@ -248,7 +248,7 @@ void rq_attach_root(struct rq *rq, struct root_domain *rd)
> >  	raw_spin_unlock_irqrestore(&rq->lock, flags);
> >  
> >  	if (old_rd)
> > -		call_rcu_sched(&old_rd->rcu, free_rootdomain);
> > +		call_rcu(&old_rd->rcu, free_rootdomain);
> >  }
> >  
> >  void sched_get_rd(struct root_domain *rd)
> > @@ -261,7 +261,7 @@ void sched_put_rd(struct root_domain *rd)
> >  	if (!atomic_dec_and_test(&rd->refcount))
> >  		return;
> >  
> > -	call_rcu_sched(&rd->rcu, free_rootdomain);
> > +	call_rcu(&rd->rcu, free_rootdomain);
> >  }
> >  
> >  static int init_rootdomain(struct root_domain *rd)
> 
> Argh, that is the rootdomain, not the regular sched-domain tree. Now
> I'll have to go audit that stuff again.
> 
> ISTR there being slightly different rules for rootdomain, and with a
> reason.
> 
> Can we hold onto this until after LPC?

This patch isn't going anywhere irrevocable until -rc5 anyway, so
no problem.  ;-)

							Thanx, Paul