Re: [PATCH] cpufreq: Fix the race condition while updating the transition_task of policy

[Liao, Chang]

Hi, Rafael

在 2023/8/28 16:58, Rafael J. Wysocki 写道:
> On Mon, Aug 28, 2023 at 10:52 AM Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
>>
>> On 28-08-23, 16:29, Liao, Chang wrote:
>>> Task B does not necessarily go to sleep when it calls wait_event(), it depends on
>>> the condition to wait for evaluate false or not. So there is a small race window
>>> where Task A already set 'transition_ongoing' to false and Task B can cross wait_event()
>>> immediately.
>>>
>>> wait_event:
>>> do {
>>>       might_sleep();
>>>       if (condition) // !transition_ongoing
>>>               break;
>>>       __wait_event();
>>> };
>>>
>>> I hope I do not miss something important in the code above.
>>
>>> Yes, if the CPU uses weak memroy model, it is possible for the instructions to be reordered.
>>> therefore, it is a good idea to insert an smb() between these two lines if there is race here.
>>
>> Maybe it would be better to do this instead ?
>>
>> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
>> index 6b52ebe5a890..f11b01b25e8d 100644
>> --- a/drivers/cpufreq/cpufreq.c
>> +++ b/drivers/cpufreq/cpufreq.c
>> @@ -455,8 +455,10 @@ void cpufreq_freq_transition_end(struct cpufreq_policy *policy,
>>                             policy->cur,
>>                             policy->cpuinfo.max_freq);
>>
>> +       spin_lock(&policy->transition_lock);
>>         policy->transition_ongoing = false;
>>         policy->transition_task = NULL;
>> +       spin_unlock(&policy->transition_lock);
>>
>>         wake_up(&policy->transition_wait);
>>  }
>>
>> --
> 
> I was about to suggest the same thing.
> 
> wake_up() is a full memory barrier only if it actually wakes up a task
> and if it doesn't do that, without the locking the other task may see
> a state in which transition_ongoing is false already and
> transition_task is still NULL regardless of the relative ordering of
> the statements before the wake_up() call.

I agree, unless the transition_ongoing and transition_task fields are updated
atomically, there is always a window where inconsistency can occur in the policy
structure.

-- 
BR
Liao, Chang