linux-kernel - Re: [PATCH] cpufreq: Fix implicit type conversion

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Date:   Fri, 29 Oct 2021 18:07:18 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Jiasheng Jiang <jiasheng@...as.ac.cn>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Benjamin Segall <bsegall@...gle.com>,
        Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpufreq: Fix implicit type conversion

On Fri, Oct 29, 2021 at 5:51 AM Jiasheng Jiang <jiasheng@...as.ac.cn> wrote:
>
> The variable 'cpu' and 'j' are defined as unsigned int.
> However in the for_each_cpu, their values are assigned to -1.
> That doesn't make sense

Yes, it does.

The binary representation of -1 is an all-ones value of the size of
int.  It is perfectly valid to store that value in an unsigned int
variable.

> and in the cpumask_next() they are implicitly type conversed to int.

However, the return type of cpumask_next() is unsigned int.

> It is universally accepted that the implicit type conversion is terrible.

I wouldn't say "terrible", but yes, it is risky when dealing with
variables of different sizes and possible sign-extensions.

In this particular case, I don't see a problem.

> Also, having the good programming custom will set an example for
> others.
> Thus, it might be better to change the definition of 'cpu' and 'j'
> from unsigned int to int.
>
> Signed-off-by: Jiasheng Jiang <jiasheng@...as.ac.cn>
> ---
>  kernel/sched/cpufreq_schedutil.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index 4f09afd..4aff4b6 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -409,7 +409,7 @@ static unsigned int sugov_next_freq_shared(struct sugov_cpu *sg_cpu, u64 time)
>         struct sugov_policy *sg_policy = sg_cpu->sg_policy;
>         struct cpufreq_policy *policy = sg_policy->policy;
>         unsigned long util = 0, max = 1;
> -       unsigned int j;
> +       int j;
>
>         for_each_cpu(j, policy->cpus) {
>                 struct sugov_cpu *j_sg_cpu = &per_cpu(sugov_cpu, j);
> @@ -746,7 +746,7 @@ static int sugov_start(struct cpufreq_policy *policy)
>  {
>         struct sugov_policy *sg_policy = policy->governor_data;
>         void (*uu)(struct update_util_data *data, u64 time, unsigned int flags);
> -       unsigned int cpu;
> +       int cpu;
>
>         sg_policy->freq_update_delay_ns = sg_policy->tunables->rate_limit_us * NSEC_PER_USEC;
>         sg_policy->last_freq_update_time        = 0;
> @@ -783,7 +783,7 @@ static int sugov_start(struct cpufreq_policy *policy)
>  static void sugov_stop(struct cpufreq_policy *policy)
>  {
>         struct sugov_policy *sg_policy = policy->governor_data;
> -       unsigned int cpu;
> +       int cpu;
>
>         for_each_cpu(cpu, policy->cpus)
>                 cpufreq_remove_update_util_hook(cpu);
> --
> 2.7.4
>