linux-kernel - Re: [RFC PATCH 08/19] cpufreq: fix warning for cpufreq_init_policy unlocked access to cpufreq_governor

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Message-ID: <20160118052336.GB30762@vireshk>
Date:	Mon, 18 Jan 2016 10:53:36 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Juri Lelli <juri.lelli@....com>
Cc:	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	peterz@...radead.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 08/19] cpufreq: fix warning for cpufreq_init_policy
 unlocked access to cpufreq_governor_list

On 14-01-16, 16:35, Juri Lelli wrote:
> But, don't we have to guarantee consinstency between multiple operations
> on cpufreq_governor_list?
> 
> In cpufreq_register_governor() we have:
> 
>  mutex_lock(&cpufreq_governor_mutex);
>  
>  governor->initialized = 0;
>  err = -EBUSY;
>  if (!find_governor(governor->name)) {
>  	err = 0;
>  	list_add(&governor->governor_list, &cpufreq_governor_list);
>  }
>  
>  mutex_unlock(&cpufreq_governor_mutex);
> 
> IIUC, find_governor and list_add have to be atomic. Couldn't someone
> slip in right after find_governor and add the same governor to the list?

Yeah, I was wrong that cpufreq_register_governor() doesn't need a
lock. We already have that in place ..

But most of the other places are really useless and shows that we
haven't implemented it well.

I would suggest that we move the lock within find_governor() and
create another find_governor_unlocked() or __find_governor() that will
be used only from cpufreq_register_governor(), with an outer lock.

Looks reasonable ?

-- 
viresh