linux-kernel - Re: [PATCH] cpufreq: Avoid using inactive policies

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Date:   Fri, 18 Nov 2016 13:20:40 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM list <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Subject: Re: [PATCH] cpufreq: Avoid using inactive policies

On Fri, Nov 18, 2016 at 4:17 AM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> On 17-11-16, 16:08, Rafael J. Wysocki wrote:
>> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>>
>> There are two places in the cpufreq core in which low-level driver
>> callbacks may be invoked for an inactive cpufreq policy, which isn't
>> guaranteed to work in general.  Both are due to possible races with
>> CPU offline.
>>
>> First, in cpufreq_get(), the policy may become inactive after
>> the check against policy->cpus in cpufreq_cpu_get() and before
>> policy->rwsem is acquired, in which case using the policy going
>> forward may not be correct.
>>
>> Second, an analogous situation is possible in cpufreq_update_policy().
>>
>> Avoid using inactive policies by adding policy_is_inactive() checks
>> to the code in the above places.
>>
>> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>> ---
>>  drivers/cpufreq/cpufreq.c |    8 +++++++-
>>  1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> Index: linux-pm/drivers/cpufreq/cpufreq.c
>> ===================================================================
>> --- linux-pm.orig/drivers/cpufreq/cpufreq.c
>> +++ linux-pm/drivers/cpufreq/cpufreq.c
>> @@ -1526,7 +1526,10 @@ unsigned int cpufreq_get(unsigned int cp
>>
>>       if (policy) {
>>               down_read(&policy->rwsem);
>> -             ret_freq = __cpufreq_get(policy);
>> +
>> +             if (!policy_is_inactive(policy))
>> +                     ret_freq = __cpufreq_get(policy);
>> +
>>               up_read(&policy->rwsem);
>>
>>               cpufreq_cpu_put(policy);
>> @@ -2265,6 +2268,9 @@ int cpufreq_update_policy(unsigned int c
>>
>>       down_write(&policy->rwsem);
>>
>> +     if (policy_is_inactive(policy))
>
> You also need to set some value to 'ret' as it is uninitialized right now.

Right, thanks!