linux-kernel - Re: [RFC PATCH 04/19] cpufreq: bring data structures close to their locks

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Date:	Tue, 12 Jan 2016 15:26:01 +0000
From:	Juri Lelli <juri.lelli@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Viresh Kumar <viresh.kumar@...aro.org>,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	rjw@...ysocki.net, mturquette@...libre.com,
	steve.muckle@...aro.org, vincent.guittot@...aro.org,
	morten.rasmussen@....com, dietmar.eggemann@....com
Subject: Re: [RFC PATCH 04/19] cpufreq: bring data structures close to their
 locks

On 12/01/16 12:36, Juri Lelli wrote:
> On 12/01/16 12:58, Peter Zijlstra wrote:
> > On Tue, Jan 12, 2016 at 11:21:25AM +0000, Juri Lelli wrote:
> > > I tried to see if something like for_each_domain() can be done, but here
> > > we use list_for_each_entry() macro. Peter, do you mean something like
> > > the following?
> > > 
> > > diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> > > index 78b1e2f..1a847a6 100644
> > > --- a/drivers/cpufreq/cpufreq.c
> > > +++ b/drivers/cpufreq/cpufreq.c
> > > @@ -39,6 +39,7 @@
> > >  static LIST_HEAD(cpufreq_governor_list);
> > >  static DEFINE_MUTEX(cpufreq_governor_mutex);
> > >  #define for_each_governor(__governor)				\
> > > +	lockdep_assert_held(&cpufreq_governor_mutex);		\
> > >  	list_for_each_entry(__governor, &cpufreq_governor_list, governor_list)
> > 
> > That fails for things like:
> > 
> > 	if (blah)
> > 		for_each_governor(...) {
> > 		}
> > 
> > which looks like valid C -- even though our Coding Style says the if
> > should have { } on.
> > 
> > I was thinking of either open coding the for statement and adding it to
> > the first statement like:
> > 
> > 	#define for_each_governor(__g) \
> > 		for (_g = list_first_entry(&cpufreq_governor_list, typeof(*_g), governor_list, lockdep_assert_held(), \
> > 		     ..... )
> > 
> > Or use something like this:
> > 
> >   lkml.kernel.org/r/20150422154212.GE3007@...ktop.Skamania.guest
> > 
> > 	#define for_each_governor(_g) \
> > 		list_for_each_entry(_g, &cpufreq_governor_list, governor_list)
> > 			if (lockdep_assert_held(..), false)
> > 				;
> > 			else
> > 
> > Which should preserve C syntax rules.
> > 
> 
> Oh, nice this! I'll try it.
> 

This second approach doesn't really play well with lockdep_assert_held
definition, right?

However, it seems I could make this work with

 #ifdef CONFIG_LOCKDEP
 #define for_each_governor(__gov)					    \
 	for (__gov = list_first_entry(&cpufreq_governor_list, 		    \
 				      typeof(*__gov), 			    \
 				      governor_list),			    \
 				WARN_ON(debug_locks &&			    \
 				!lockdep_is_held(&cpufreq_governor_mutex)); \
 	     &__gov->governor_list != (&cpufreq_governor_list);		    \
 	     __gov = list_next_entry(__gov, governor_list))
 #else /* !CONFIG_LOCKDEP */
 #define for_each_governor(__gov)					    \
 	list_for_each_entry(__gov, &cpufreq_governor_list, governor_list)
 #endif /* CONFIG_LOCKDEP */

Thanks,

- Juri