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: <20160114163511.GA4246@e106622-lin>
Date:	Thu, 14 Jan 2016 16:35:11 +0000
From:	Juri Lelli <juri.lelli@....com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
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 13/01/16 11:37, Viresh Kumar wrote:
> On 12-01-16, 15:52, Juri Lelli wrote:
> > Other users (i.e., cpufreq_parse_governor and cpufreq_register_governor)
> > needs to take the mutex externally. So, we need to unify this behaviour.
> 
> No they don't have to.
> 
> And that's why I have been saying that we better nail down the exact
> thing the mutex is supposed to protect.
> 
> There can be two cases here:
> - It protects the governor list, in that case we can move it to
>   find_governor().
> - It guarantees that the governor pointer stays valid: That's not true
>   as we are using the governor pointer outside of the lock.
> 
> And so I said, "No they don't have to" :)
> 

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?

Thanks,

- Juri