linux-kernel - Re: [PATCH]cpuidle: preventive check in cpuidle

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Message-ID: <CAJZ5v0jH=DqUSFT8yAt+M=_N0Qn-ou0dPmqK7DwG5S22jZ1R+A@mail.gmail.com>
Date:   Wed, 27 Dec 2017 03:30:02 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     gaurav jindal <gauravjindal1104@...il.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH]cpuidle: preventive check in cpuidle_select against crash

On Wed, Dec 27, 2017 at 2:57 AM, gaurav jindal
<gauravjindal1104@...il.com> wrote:
> On Wed, Dec 27, 2017 at 01:42:58AM +0100, Rafael J. Wysocki wrote:
>> On Tue, Dec 26, 2017 at 8:26 AM, gaurav jindal
>> <gauravjindal1104@...il.com> wrote:
>> > When selecting the idle state using cpuidle_select, there is no
>> > check on cpuidle_curr_governor. In cpuidle_switch_governor,
>> > cpuidle_currr_governor can be set to NULL to specify "disabled".
>>
>> How exactly?
>
> In cpuidle_switch_governor:
>
> /**
>  * cpuidle_switch_governor - changes the governor
>  * @gov: the new target governor
>  *
>  * NOTE: "gov" can be NULL to specify disabled
>  * Must be called with cpuidle_lock acquired.
>  */
> int cpuidle_switch_governor(struct cpuidle_governor *gov)
> {
>         struct cpuidle_device *dev;
>
>         if (gov == cpuidle_curr_governor)
>                 return 0;
>
>         cpuidle_uninstall_idle_handler();
>
>         if (cpuidle_curr_governor) {
>                 list_for_each_entry(dev, &cpuidle_detected_devices, device_list)
>                         cpuidle_disable_device(dev);
>         }
>
>         cpuidle_curr_governor = gov;
>
> This allows to set the cpuidle_switch_governor as NULL. Although there is no
> current code flow leading here, but it has a potential for bug in future. So
> may be better to have prevention.

Or maybe not.

Why don't you make cpuidle_switch_governor() check the argument
against NULL instead?