Re: [PATCH v2] cpufreq: Fix the race condition while updating the transition_task of policy

[Viresh Kumar]

On 29-08-23, 07:03, Liao Chang wrote:
> The field 'transition_task' of policy structure is used to track the
> task which is performing the frequency transition. Using this field to
> print a warning once detect a case where the same task is calling
> _begin() again before completing the preivous frequency transition via
> the _end().
> 
> However, there is a potential race condition in _end() and _begin() APIs
> while updating the field 'transition_task' of policy, the scenario is
> depicted below:
> 
>              Task A                            Task B
> 
>         /* 1st freq transition */
>         Invoke _begin() {
>                 ...
>                 ...
>         }
>                                         /* 2nd freq transition */
>                                         Invoke _begin() {
>                                                 ... //waiting for A to
>                                                 ... //clear
>                                                 ... //transition_ongoing
>                                                 ... //in _end() for
>                                                 ... //the 1st transition
>                                                         |
>         Change the frequency                            |
>                                                         |
>         Invoke _end() {                                 |
>                 ...                                     |
>                 ...                                     |
>                 transition_ongoing = false;             V
>                                                 transition_ongoing = true;
>                                                 transition_task = current;
>                 transition_task = NULL;
>                 ... //A overwrites the task
>                 ... //performing the transition
>                 ... //result in error warning.
>         }
> 
> To fix this race condition, the transition_lock of policy structure is
> now acquired before updating policy structure in _end() API. Which ensure
> that only one task can update the 'transition_task' field at a time.
> 
> Link: https://lore.kernel.org/all/b3c61d8a-d52d-3136-fbf0-d1de9f1ba411@xxxxxxxxxx/
> Fixes: ca654dc3a93d ("cpufreq: Catch double invocations of cpufreq_freq_transition_begin/end")
> Signed-off-by: Liao Chang <liaochang1@xxxxxxxxxx>
> ---
>  drivers/cpufreq/cpufreq.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 26fa99d46684..5c09a4936699 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);
>  }

Acked-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>

-- 
viresh