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)
 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 
> CC: Rafael J. Wysocki 
> CC: Peter Zijlstra 
> CC: Ingo Molnar 
> CC: Patrick Bellasi 
> CC: Juri Lelli 
> Cc: Luca Abeni 
> CC: Todd Kjos 
> CC: clau...@evidence.eu.com
> CC: kernel-t...@android.com
> CC: linux...@vger.kernel.org
> Signed-off-by: Joel Fernandes (Google) 
> ---
> 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(_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(_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.

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)
 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 
> CC: Rafael J. Wysocki 
> CC: Peter Zijlstra 
> CC: Ingo Molnar 
> CC: Patrick Bellasi 
> CC: Juri Lelli 
> Cc: Luca Abeni 
> CC: Todd Kjos 
> CC: clau...@evidence.eu.com
> CC: kernel-t...@android.com
> CC: linux...@vger.kernel.org
> Signed-off-by: Joel Fernandes (Google) 
> ---
> 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(_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(_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.

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 
> CC: Rafael J. Wysocki 
> CC: Peter Zijlstra 
> CC: Ingo Molnar 
> CC: Patrick Bellasi 
> CC: Juri Lelli 
> Cc: Luca Abeni 
> CC: Todd Kjos 
> CC: clau...@evidence.eu.com
> CC: kernel-t...@android.com
> CC: linux...@vger.kernel.org
> Signed-off-by: Joel Fernandes (Google) 
> ---
> 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(_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

[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 
> CC: Rafael J. Wysocki 
> CC: Peter Zijlstra 
> CC: Ingo Molnar 
> CC: Patrick Bellasi 
> CC: Juri Lelli 
> Cc: Luca Abeni 
> CC: Todd Kjos 
> CC: clau...@evidence.eu.com
> CC: kernel-t...@android.com
> CC: linux...@vger.kernel.org
> Signed-off-by: Joel Fernandes (Google) 
> ---
> 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(_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

[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

[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

[..] 

[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 
CC: Rafael J. Wysocki 
CC: Peter Zijlstra 
CC: Ingo Molnar 
CC: Patrick Bellasi 
CC: Juri Lelli 
Cc: Luca Abeni 
CC: Todd Kjos 
CC: clau...@evidence.eu.com
CC: kernel-t...@android.com
CC: linux...@vger.kernel.org
Signed-off-by: Joel Fernandes (Google) 
---
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(_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(_policy->update_lock, flags);
 
-- 
2.17.0.441.gb46fe60e1d-goog

[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 
CC: Rafael J. Wysocki 
CC: Peter Zijlstra 
CC: Ingo Molnar 
CC: Patrick Bellasi 
CC: Juri Lelli 
Cc: Luca Abeni 
CC: Todd Kjos 
CC: clau...@evidence.eu.com
CC: kernel-t...@android.com
CC: linux...@vger.kernel.org
Signed-off-by: Joel Fernandes (Google) 
---
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(_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(_policy->update_lock, flags);
 
-- 
2.17.0.441.gb46fe60e1d-goog