linux-kernel - Re: [RFC PATCH 11/19] cpufreq: assert policy->rwsem is held in __cpufreq

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Date:	Fri, 29 Jan 2016 16:33:39 -0800
From:	Saravana Kannan <skannan@...eaurora.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
CC:	Juri Lelli <juri.lelli@....com>, 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 11/19] cpufreq: assert policy->rwsem is held in __cpufreq_governor

On 01/12/2016 02:20 AM, Viresh Kumar wrote:
> On 11-01-16, 17:35, Juri Lelli wrote:
>> __cpufreq_governor works on policy, so policy->rwsem has to be held.
>> Add assertion for such condition.
>>
>> Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>
>> Cc: Viresh Kumar <viresh.kumar@...aro.org>
>> Signed-off-by: Juri Lelli <juri.lelli@....com>
>> ---
>>   drivers/cpufreq/cpufreq.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
>> index f1f9fbc..e7fc5c9 100644
>> --- a/drivers/cpufreq/cpufreq.c
>> +++ b/drivers/cpufreq/cpufreq.c
>> @@ -1950,6 +1950,9 @@ static int __cpufreq_governor(struct cpufreq_policy *policy,
>>   	/* Don't start any governor operations if we are entering suspend */
>>   	if (cpufreq_suspended)
>>   		return 0;
>> +
>> +	lockdep_assert_held(&policy->rwsem);
>> +
>
> We had an ABBA problem with the EXIT governor callback and so this
> rwsem is dropped just before that from set_policy()..
>
> commit 955ef4833574 ("cpufreq: Drop rwsem lock around
> CPUFREQ_GOV_POLICY_EXIT")
>

AFAIR, the ABBA issue was between the sysfs lock and the policy lock. 
The fix for that issue should not be dropping the lock around 
POLICY_EXIT. The proper fix is to have the governor "export" the 
attributes it wants to add/remove and have the cpufreq framework do the 
adding/removing of the attributes from sysfs for the governor.

Thanks,
Saravana

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