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Message-ID: <6583cf83-3598-7b7c-4753-611951c5d09b@huawei.com>
Date:   Mon, 28 Aug 2023 17:29:04 +0800
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Subject: Re: [PATCH] cpufreq: Fix the race condition while updating the
 transition_task of policy
To:     Viresh Kumar <viresh.kumar@linaro.org>
CC:     <rafael@kernel.org>, <srivatsa.bhat@linux.vnet.ibm.com>,
        <linux-pm@vger.kernel.org>, <linux-kernel@vger.kernel.org>
References: <20230826095836.1138608-1-liaochang1@huawei.com>
 <20230828072347.ly23mbptu3yw4zkv@vireshk-i7>
 <bcac1b3c-a2f7-7f18-4c2f-5cf27f4e89b5@huawei.com>
 <20230828085248.sz6aljr5aln7j435@vireshk-i7>
From:   "Liao, Chang" <liaochang1@huawei.com>
In-Reply-To: <20230828085248.sz6aljr5aln7j435@vireshk-i7>
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在 2023/8/28 16:52, Viresh Kumar 写道:
> 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);

I think it is more straightforward, I will use it in next revision.

Thanks.

> 
>         wake_up(&policy->transition_wait);
>  }
> 

-- 
BR
Liao, Chang