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

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Date:	Tue, 12 Jan 2016 12:58:43 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Juri Lelli <juri.lelli@....com>
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 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.