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

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Date:   Fri, 18 Nov 2016 13:25:00 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        "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 1:20 PM, Rafael J. Wysocki <rafael@...nel.org> wrote:
> 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!

Which doesn't matter too much, though, because none of the callers
actually checks the return value. :-)

acpi_processor_ppc_has_changed() returns it further, but none of the
callers of that checks the value returned by it.

I guess I'll post a cleanup on top of this ...

Thanks,
Rafael