[PATCH RFC] schedutil: Address the r/w ordering race in kthread

[PATCH RFC] schedutil: Address the r/w ordering race in kthread

[Joel Fernandes (Google)]

Currently there is a race in schedutil code for slow-switch single-CPU
systems. Fix it by enforcing ordering the write to work_in_progress to
happen before the read of next_freq.

Kthread                                       Sched update

sugov_work()				      sugov_update_single()

      lock();
      // The CPU is free to rearrange below
      // two in any order, so it may clear
      // the flag first and then read next
      // freq. Lets assume it does.
      work_in_progress = false

                                               if (work_in_progress)
                                                     return;

                                               sg_policy->next_freq = 0;
      freq = sg_policy->next_freq;
                                               sg_policy->next_freq = real-freq;
      unlock();

Reported-by: Viresh Kumar <viresh.kumar@linaro.org>
CC: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
CC: Peter Zijlstra <peterz@infradead.org>
CC: Ingo Molnar <mingo@redhat.com>
CC: Patrick Bellasi <patrick.bellasi@arm.com>
CC: Juri Lelli <juri.lelli@redhat.com>
Cc: Luca Abeni <luca.abeni@santannapisa.it>
CC: Todd Kjos <tkjos@google.com>
CC: claudio@evidence.eu.com
CC: kernel-team@android.com
CC: linux-pm@vger.kernel.org
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
I split this into separate patch, because this race can also happen in
mainline.

 kernel/sched/cpufreq_schedutil.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index 5c482ec38610..ce7749da7a44 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -401,6 +401,13 @@ static void sugov_work(struct kthread_work *work)
 	 */
 	raw_spin_lock_irqsave(&sg_policy->update_lock, flags);
 	freq = sg_policy->next_freq;
+
+	/*
+	 * sugov_update_single can access work_in_progress without update_lock,
+	 * make sure next_freq is read before work_in_progress is set.
+	 */
+	smp_mb();
+
 	sg_policy->work_in_progress = false;
 	raw_spin_unlock_irqrestore(&sg_policy->update_lock, flags);
 
-- 
2.17.0.441.gb46fe60e1d-goog

Re: [PATCH RFC] schedutil: Address the r/w ordering race in kthread

[Joel Fernandes]

On Tue, May 22, 2018 at 04:50:28PM -0700, Joel Fernandes (Google) wrote:
> Currently there is a race in schedutil code for slow-switch single-CPU
> systems. Fix it by enforcing ordering the write to work_in_progress to
> happen before the read of next_freq.

Aargh, s/before/after/.

Commit log has above issue but code is Ok. Should I resend this patch or
are there any additional comments? thanks!

 - Joel

[..] 

Re: [PATCH RFC] schedutil: Address the r/w ordering race in kthread

[Juri Lelli]

Hi Joel,

On 22/05/18 16:50, Joel Fernandes (Google) wrote:
> Currently there is a race in schedutil code for slow-switch single-CPU
> systems. Fix it by enforcing ordering the write to work_in_progress to
> happen before the read of next_freq.
> 
> Kthread                                       Sched update
> 
> sugov_work()				      sugov_update_single()
> 
>       lock();
>       // The CPU is free to rearrange below
>       // two in any order, so it may clear
>       // the flag first and then read next
>       // freq. Lets assume it does.
>       work_in_progress = false
> 
>                                                if (work_in_progress)
>                                                      return;
> 
>                                                sg_policy->next_freq = 0;
>       freq = sg_policy->next_freq;
>                                                sg_policy->next_freq = real-freq;
>       unlock();
> 
> Reported-by: Viresh Kumar <viresh.kumar@linaro.org>
> CC: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> CC: Peter Zijlstra <peterz@infradead.org>
> CC: Ingo Molnar <mingo@redhat.com>
> CC: Patrick Bellasi <patrick.bellasi@arm.com>
> CC: Juri Lelli <juri.lelli@redhat.com>
> Cc: Luca Abeni <luca.abeni@santannapisa.it>
> CC: Todd Kjos <tkjos@google.com>
> CC: claudio@evidence.eu.com
> CC: kernel-team@android.com
> CC: linux-pm@vger.kernel.org
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
> I split this into separate patch, because this race can also happen in
> mainline.
> 
>  kernel/sched/cpufreq_schedutil.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index 5c482ec38610..ce7749da7a44 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -401,6 +401,13 @@ static void sugov_work(struct kthread_work *work)
>  	 */
>  	raw_spin_lock_irqsave(&sg_policy->update_lock, flags);
>  	freq = sg_policy->next_freq;
> +
> +	/*
> +	 * sugov_update_single can access work_in_progress without update_lock,
> +	 * make sure next_freq is read before work_in_progress is set.

s/set/reset/

> +	 */
> +	smp_mb();
> +

Also, doesn't this need a corresponding barrier (I guess in
sugov_should_update_freq)? That being a wmb and this a rmb?

Best,

- Juri

Re: [PATCH RFC] schedutil: Address the r/w ordering race in kthread

[Rafael J. Wysocki]

On Wed, May 23, 2018 at 1:50 AM, Joel Fernandes (Google)
<joelaf@google.com> wrote:
> Currently there is a race in schedutil code for slow-switch single-CPU
> systems. Fix it by enforcing ordering the write to work_in_progress to
> happen before the read of next_freq.
>
> Kthread                                       Sched update
>
> sugov_work()                                  sugov_update_single()
>
>       lock();
>       // The CPU is free to rearrange below
>       // two in any order, so it may clear
>       // the flag first and then read next
>       // freq. Lets assume it does.
>       work_in_progress = false
>
>                                                if (work_in_progress)
>                                                      return;
>
>                                                sg_policy->next_freq = 0;
>       freq = sg_policy->next_freq;
>                                                sg_policy->next_freq = real-freq;
>       unlock();
>
> Reported-by: Viresh Kumar <viresh.kumar@linaro.org>
> CC: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> CC: Peter Zijlstra <peterz@infradead.org>
> CC: Ingo Molnar <mingo@redhat.com>
> CC: Patrick Bellasi <patrick.bellasi@arm.com>
> CC: Juri Lelli <juri.lelli@redhat.com>
> Cc: Luca Abeni <luca.abeni@santannapisa.it>
> CC: Todd Kjos <tkjos@google.com>
> CC: claudio@evidence.eu.com
> CC: kernel-team@android.com
> CC: linux-pm@vger.kernel.org
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
> I split this into separate patch, because this race can also happen in
> mainline.
>
>  kernel/sched/cpufreq_schedutil.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index 5c482ec38610..ce7749da7a44 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -401,6 +401,13 @@ static void sugov_work(struct kthread_work *work)
>          */
>         raw_spin_lock_irqsave(&sg_policy->update_lock, flags);
>         freq = sg_policy->next_freq;
> +
> +       /*
> +        * sugov_update_single can access work_in_progress without update_lock,
> +        * make sure next_freq is read before work_in_progress is set.
> +        */
> +       smp_mb();
> +

This requires a corresponding barrier somewhere else.

>         sg_policy->work_in_progress = false;
>         raw_spin_unlock_irqrestore(&sg_policy->update_lock, flags);
>
> --

Also, as I said I actually would prefer to use the spinlock in the
one-CPU case when the kthread is used.

I'll have a patch for that shortly.