Date:   Wed, 11 May 2022 21:12:32 +0800
From:   Schspa Shi <schspa@...il.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>
Subject: Re: [PATCH v3] cpufreq: fix race on cpufreq online

Viresh Kumar <viresh.kumar@...aro.org> writes:

> On 11-05-22, 16:10, Schspa Shi wrote:
>> Viresh Kumar <viresh.kumar@...aro.org> writes:
>> > I am not sure, but maybe there were issues in calling init() with rwsem held, as
>> > it may want to call some API from there.
>> >
>>
>> I have checked all the init() implement of the fellowing files, It should be OK.
>> Function find command:
>>   ag "init[\s]+=" drivers/cpufreq
>>
>> All the init() implement only initialize policy object without holding this lock
>> and won't call cpufreq APIs need to hold this lock.
>
> Okay, we can see if someone complains later then :)
>
>> > I don't think you can do that safely. offline() or exit() may depend on
>> > policy->cpus being set to all CPUs.
>> OK, I will move this after exit(). and there will be no effect with those
>> two APIs. But policy->cpus must be clear before release policy->rwsem.
>
> Hmm, I don't think depending on the values of policy->cpus is a good idea to be
> honest. This design is inviting bugs to come in at another place. We need a
> clear flag for this, a new flag or something like policy_list.
>
> Also I see the same bug happening while the policy is removed. The kobject is
> put after the rwsem is dropped.
>
>> >  static inline bool policy_is_inactive(struct cpufreq_policy *policy)
>> >  {
>> > -     return cpumask_empty(policy->cpus);
>> > +     return unlikely(cpumask_empty(policy->cpus) ||
>> > +                     list_empty(&policy->policy_list));
>> >  }
>> >
>>
>> I don't think this fully solves my problem.
>> 1. There is some case which cpufreq_online failed after the policy is added to
>>    cpufreq_policy_list.
>
> And I missed that :(
>
>> 2. policy->policy_list is not protected by &policy->rwsem, and we
>> can't relay on this to
>>    indict the policy is fine.
>
> Ahh..
>
>> >From this point of view, we can fix this problem through the state of
>> this linked list.
>> But the above two problems need to be solved first.
>
> I feel overriding policy_list for this is going to make it complex/messy.
>

Yes, I agree with it.

> Maybe something like this then:
>
> -------------------------8<-------------------------
>
> From dacc8d09d4d7b3d9a8bca8d78fc72199c16dc4a5 Mon Sep 17 00:00:00 2001
> Message-Id: <dacc8d09d4d7b3d9a8bca8d78fc72199c16dc4a5.1652271581.git.viresh.kumar@...aro.org>
> From: Viresh Kumar <viresh.kumar@...aro.org>
> Date: Wed, 11 May 2022 09:13:26 +0530
> Subject: [PATCH] cpufreq: Allow sysfs access only for active policies
>
> It is currently possible, in a corner case, to access the sysfs files
> and reach show_cpuinfo_cur_freq(), etc, for a partly initialized policy.
>
> This can happen for example if cpufreq_online() fails after adding the
> sysfs files, which are immediately accessed by another process. There
> can easily be other such cases, which aren't identified yet, like while
> the policy is getting freed.
>
> Process A:                                    Process B
>
> cpufreq_online()
>   down_write(&policy->rwsem);
>   if (new_policy) {
>     ret = cpufreq_add_dev_interface(policy);
>     /* This fails after adding few files */
>     if (ret)
>       goto out_destroy_policy;
>
>     ...
>   }
>
>   ...
>
> out_destroy_policy:
>   ...
>   up_write(&policy->rwsem);
>                                               /*
>                                                * This will end up accessing the policy
>                                                * which isn't fully initialized.
>                                                */
>                                               show_cpuinfo_cur_freq()
>
> if (cpufreq_driver->offline)
>     cpufreq_driver->offline(policy);
>
>   if (cpufreq_driver->exit)
>     cpufreq_driver->exit(policy);
>
>   cpufreq_policy_free(policy);
>
> Fix these by checking in show/store if the policy is sysfs ready or not.
>
> Reported-by: Schspa Shi <schspa@...il.com>
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> ---
>  drivers/cpufreq/cpufreq.c | 18 ++++++++++++++----
>  include/linux/cpufreq.h   |  3 +++
>  2 files changed, 17 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index c8bf6c68597c..65c2bbcf555d 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -948,13 +948,14 @@ static ssize_t show(struct kobject *kobj, struct attribute *attr, char *buf)
>  {
>       struct cpufreq_policy *policy = to_policy(kobj);
>       struct freq_attr *fattr = to_attr(attr);
> -     ssize_t ret;
> +     ssize_t ret = -EBUSY;
>
>       if (!fattr->show)
>               return -EIO;
>
>       down_read(&policy->rwsem);
> -     ret = fattr->show(policy, buf);
> +     if (policy->sysfs_ready)
> +             ret = fattr->show(policy, buf);
>       up_read(&policy->rwsem);
>
>       return ret;
> @@ -965,7 +966,7 @@ static ssize_t store(struct kobject *kobj, struct attribute *attr,
>  {
>       struct cpufreq_policy *policy = to_policy(kobj);
>       struct freq_attr *fattr = to_attr(attr);
> -     ssize_t ret = -EINVAL;
> +     ssize_t ret = -EBUSY;
>
>       if (!fattr->store)
>               return -EIO;
> @@ -979,7 +980,8 @@ static ssize_t store(struct kobject *kobj, struct attribute *attr,
>
>       if (cpu_online(policy->cpu)) {
>               down_write(&policy->rwsem);
> -             ret = fattr->store(policy, buf, count);
> +             if (policy->sysfs_ready)
> +                     ret = fattr->store(policy, buf, count);
>               up_write(&policy->rwsem);
>       }
>
> @@ -1280,6 +1282,11 @@ static void cpufreq_policy_free(struct cpufreq_policy *policy)
>       unsigned long flags;
>       int cpu;
>
> +     /* Disallow sysfs interactions now */
> +     down_write(&policy->rwsem);
> +     policy->sysfs_ready = false;
> +     up_write(&policy->rwsem);
> +
>       /* Remove policy from list */
>       write_lock_irqsave(&cpufreq_driver_lock, flags);
>       list_del(&policy->policy_list);
> @@ -1516,6 +1523,9 @@ static int cpufreq_online(unsigned int cpu)
>               goto out_destroy_policy;
>       }
>
> +     /* We can allow sysfs interactions now */
> +     policy->sysfs_ready = true;
> +
>       up_write(&policy->rwsem);
>
>       kobject_uevent(&policy->kobj, KOBJ_ADD);
> diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
> index 35c7d6db4139..7e4384e535fd 100644
> --- a/include/linux/cpufreq.h
> +++ b/include/linux/cpufreq.h
> @@ -101,6 +101,9 @@ struct cpufreq_policy {
>        */
>       struct rw_semaphore     rwsem;
>
> +     /* Policy is ready for sysfs interactions */
> +     bool                    sysfs_ready;
> +

Do we need to add this flag to some APIs like
  unsigned int cpufreq_get(unsigned int cpu);
  void refresh_frequency_limits(struct cpufreq_policy *policy);
too ?

But if we made this change it seems to have the same meaning as
policy_is_inactive.

>       /*
>        * Fast switch flags:
>        * - fast_switch_possible should be set by the driver if it can

---
BRs

Schspa Shi

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