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Message-ID: <20230828072347.ly23mbptu3yw4zkv@vireshk-i7>
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
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