linux-kernel - Re: [PATCH] cpufreq: Fix the race condition while updating the transition

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Date:   Mon, 28 Aug 2023 12:53:47 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Liao Chang <liaochang1@...wei.com>
Cc:     rafael@...nel.org, srivatsa.bhat@...ux.vnet.ibm.com,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cpufreq: Fix the race condition while updating the
 transition_task of policy

On 26-08-23, 09:58, 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;

Task B here won't move ahead until "wake_up(&policy->transition_wait)"
is called, isn't it ?

Also I think the CPU is free to change the order of the two
instructions and so this commit won't make a difference. Also I don't
feel there is a race here as wake_up() hasn't happened.

>                 transition_task = NULL;
>                 ... //A overwrites the task
>                 ... //performing the transition
>                 ... //result in error warning.
>         }
> 
> To fix this race condition, the order of the updates to the
> 'transition_task' and 'transition_ongoing' fields has been changed, the
> 'transition_task' field is now cleared before the 'transition_ongoing'
> field, which ensure that only one task can update the 'transition_task'
> field at a time.
> 
> Fixes: ca654dc3a93d ("cpufreq: Catch double invocations of cpufreq_freq_transition_begin/end")
> Signed-off-by: Liao Chang <liaochang1@...wei.com>
> ---
>  drivers/cpufreq/cpufreq.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index a757f90aa9d6..f8eb6dde57f2 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -455,8 +455,8 @@ void cpufreq_freq_transition_end(struct cpufreq_policy *policy,
>  			    policy->cur,
>  			    policy->cpuinfo.max_freq);
>  
> -	policy->transition_ongoing = false;
>  	policy->transition_task = NULL;
> +	policy->transition_ongoing = false;
>  
>  	wake_up(&policy->transition_wait);
>  }
> -- 
> 2.34.1

-- 
viresh