Re: [PATCH V3 1/3] cpufreq: Fix locking issues with governors

[Quentin Perret <qperret@google.com>]

On Friday 26 Jun 2020 at 09:21:42 (+0530), Viresh Kumar wrote:
> 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 0128de3603df..e798a1193bdf 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -621,6 +621,24 @@ 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))
> @@ -640,28 +658,14 @@ 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);
>  }
>  
>  /**
> @@ -815,12 +819,14 @@ 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;
> @@ -1058,11 +1064,14 @@ static int cpufreq_init_policy(struct cpufreq_policy *policy)
>  	struct cpufreq_governor *def_gov = cpufreq_default_governor();
>  	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 (def_gov) {
> @@ -1089,7 +1098,11 @@ 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);

Nit: I think you could safely do

	if (gov)
		module_put(gov->owner);

and get rid of 'put_governor', given that try_module_get() and
module_put() are nops if owner is NULL (which is guaranteed for
the result of cpufreq_default_governor() as it is builtin).

But other than that:

Reviewed-by: Quentin Perret <qperret@google.com>

Thanks!