Re: [PATCH v2 2/2] cpufreq: Specify default governor on command line

["Rafael J. Wysocki" <rafael@kernel.org>]

On Thu, Jun 25, 2020 at 10:50 AM Viresh Kumar <viresh.kumar@linaro.org> wrote:
>
> On 24-06-20, 16:32, Quentin Perret wrote:
> > Right, but I must admit that, looking at this more, I'm getting a bit
> > confused with the overall locking for governors :/
> >
> > When in cpufreq_init_policy() we find a governor using
> > find_governor(policy->last_governor), what guarantees this governor is
> > not concurrently unregistered? That is, what guarantees this governor
> > doesn't go away between that find_governor() call, and the subsequent
> > call to try_module_get() in cpufreq_set_policy() down the line?
> >
> > Can we somewhat assume that whatever governor is referred to by
> > policy->last_governor will have a non-null refcount? Or are the
> > cpufreq_online() and cpufreq_unregister_governor() path mutually
> > exclusive? Or is there something else?
>
> This should be sufficient to fix pending issues I believe. Based over your
> patches.

LGTM, but can you post it in a new thread to let Patchwork pick it up?

> -------------------------8<-------------------------
> From: Viresh Kumar <viresh.kumar@linaro.org>
> Date: Thu, 25 Jun 2020 13:15:23 +0530
> Subject: [PATCH] cpufreq: Fix locking issues with governors
>
> The locking around governors handling isn't adequate currently. The list
> of governors should never be traversed without locking in place. Also we
> must make sure the governor isn't removed while it is still referenced
> by code.
>
> Reported-by: Quentin Perret <qperret@google.com>
> Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
> ---
>  drivers/cpufreq/cpufreq.c | 59 ++++++++++++++++++++++++---------------
>  1 file changed, 36 insertions(+), 23 deletions(-)
>
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 4b1a5c0173cf..dad6b85f4c89 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -624,6 +624,24 @@ static struct cpufreq_governor *find_governor(const char *str_governor)
>         return NULL;
>  }
>
> +static struct cpufreq_governor *get_governor(const char *str_governor)
> +{
> +       struct cpufreq_governor *t;
> +
> +       mutex_lock(&cpufreq_governor_mutex);
> +       t = find_governor(str_governor);
> +       if (!t)
> +               goto unlock;
> +
> +       if (!try_module_get(t->owner))
> +               t = NULL;
> +
> +unlock:
> +       mutex_unlock(&cpufreq_governor_mutex);
> +
> +       return t;
> +}
> +
>  static unsigned int cpufreq_parse_policy(char *str_governor)
>  {
>         if (!strncasecmp(str_governor, "performance", CPUFREQ_NAME_LEN))
> @@ -643,28 +661,14 @@ static struct cpufreq_governor *cpufreq_parse_governor(char *str_governor)
>  {
>         struct cpufreq_governor *t;
>
> -       mutex_lock(&cpufreq_governor_mutex);
> -
> -       t = find_governor(str_governor);
> -       if (!t) {
> -               int ret;
> -
> -               mutex_unlock(&cpufreq_governor_mutex);
> -
> -               ret = request_module("cpufreq_%s", str_governor);
> -               if (ret)
> -                       return NULL;
> -
> -               mutex_lock(&cpufreq_governor_mutex);
> +       t = get_governor(str_governor);
> +       if (t)
> +               return t;
>
> -               t = find_governor(str_governor);
> -       }
> -       if (t && !try_module_get(t->owner))
> -               t = NULL;
> -
> -       mutex_unlock(&cpufreq_governor_mutex);
> +       if (request_module("cpufreq_%s", str_governor))
> +               return NULL;
>
> -       return t;
> +       return get_governor(str_governor);
>  }
>
>  /**
> @@ -818,12 +822,14 @@ static ssize_t show_scaling_available_governors(struct cpufreq_policy *policy,
>                 goto out;
>         }
>
> +       mutex_lock(&cpufreq_governor_mutex);
>         for_each_governor(t) {
>                 if (i >= (ssize_t) ((PAGE_SIZE / sizeof(char))
>                     - (CPUFREQ_NAME_LEN + 2)))
> -                       goto out;
> +                       break;
>                 i += scnprintf(&buf[i], CPUFREQ_NAME_PLEN, "%s ", t->name);
>         }
> +       mutex_unlock(&cpufreq_governor_mutex);
>  out:
>         i += sprintf(&buf[i], "\n");
>         return i;
> @@ -1060,11 +1066,14 @@ static int cpufreq_init_policy(struct cpufreq_policy *policy)
>  {
>         struct cpufreq_governor *gov = NULL;
>         unsigned int pol = CPUFREQ_POLICY_UNKNOWN;
> +       bool put_governor = false;
> +       int ret;
>
>         if (has_target()) {
>                 /* Update policy governor to the one used before hotplug. */
> -               gov = find_governor(policy->last_governor);
> +               gov = get_governor(policy->last_governor);
>                 if (gov) {
> +                       put_governor = true;
>                         pr_debug("Restoring governor %s for cpu %d\n",
>                                  policy->governor->name, policy->cpu);
>                 } else if (default_governor) {
> @@ -1091,7 +1100,11 @@ static int cpufreq_init_policy(struct cpufreq_policy *policy)
>                         return -ENODATA;
>         }
>
> -       return cpufreq_set_policy(policy, gov, pol);
> +       ret = cpufreq_set_policy(policy, gov, pol);
> +       if (put_governor)
> +               module_put(gov->owner);
> +
> +       return ret;
>  }
>
>  static int cpufreq_add_policy_cpu(struct cpufreq_policy *policy, unsigned int cpu)