Re: [PATCH] thermal: fix race condition when updating cooling device

[Michele DiGiorgio]

Hi Eduardo, Rui,

On 20/07/16 10:46, Michele Di Giorgio wrote:
> When multiple thermal zones are bound to the same cooling device, multiple
> kernel threads may want to update the cooling device state by calling
> thermal_cdev_update(). Having cdev not protected by a mutex can lead to a race
> condition. Consider the following situation with two kernel threads k1 and k2:
>
>           Thread k1                           Thread k2
>                                      ||
>                                      ||  call thermal_cdev_update()
>                                      ||      ...
>                                      ||      set_cur_state(cdev, target);
>      call power_actor_set_power()    ||
>          ...                         ||
>          instance->target = state;   ||
>          cdev->updated = false;      ||
>                                      ||      cdev->updated = true;
>                                      ||      // completes execution
>      call thermal_cdev_update()      ||
>          // cdev->updated == true    ||
>          return;                     ||
>                                      \/
>                                      time
>
> k2 has already looped through the thermal instances looking for the deepest
> cooling device state and is preempted right before setting cdev->updated to
> true. Now, k1 runs, modifies the thermal instance state and sets cdev->updated
> to false. Then, k1 is preempted and k2 continues the execution by setting
> cdev->updated to true, therefore preventing k1 from performing the update.
> Notice that this is not an issue if k2 looks at the instance->target modified by
> k1 "after" it is assigned by k1. In fact, in this case the update will happen
> anyway and k1 can safely return immediately from thermal_cdev_update().
>
> This may lead to a situation where a thermal governor never updates the cooling
> device. For example, this is the case for the step_wise governor: when calling
> the function thermal_zone_trip_update(), the governor may always get a new state
> equal to the old one (which, however, wasn't notified to the cooling device) and
> will therefore skip the update.
>
> CC: Zhang Rui <rui.zhang@xxxxxxxxx>
> CC: Eduardo Valentin <edubezval@xxxxxxxxx>
> CC: Peter Feuerer <peter@xxxxxxxx>
> Reported-by: Toby Huang <toby.huang@xxxxxxx>
> Signed-off-by: Michele Di Giorgio <michele.digiorgio@xxxxxxx>
> ---
> Protecting only the assignment of cdev->updated with mutexes may look
> suspicious, but it is necessary to guarantee synchronization and avoiding the
> situation described in the commit message.
>
> There are other two possible solutions.
>
> Moving the cdev->lock mutex outside thermal_cdev_update() and protect both the
> assignment and the function. This would work, but will probably cause many
> issues when updating all the modules that use thermal_cdev_update().
>
> The other solution is to make cdev->updated an atomic_t, change the if
> condition to an atomic_cmpxchg and extend the critical section to include the
> call to cdev->ops->set_cur_state().
>
>   drivers/thermal/fair_share.c      |  2 ++
>   drivers/thermal/gov_bang_bang.c   |  2 ++
>   drivers/thermal/power_allocator.c |  2 ++
>   drivers/thermal/step_wise.c       |  2 ++
>   drivers/thermal/thermal_core.c    | 10 +++++++---
>   5 files changed, 15 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/thermal/fair_share.c b/drivers/thermal/fair_share.c
> index 34fe365..68bd1b5 100644
> --- a/drivers/thermal/fair_share.c
> +++ b/drivers/thermal/fair_share.c
> @@ -116,7 +116,9 @@ static int fair_share_throttle(struct thermal_zone_device *tz, int trip)
>               instance->target = get_target_state(tz, cdev, percentage,
>                                                   cur_trip_level);
>
> +             mutex_lock(&instance->cdev->lock);
>               instance->cdev->updated = false;
> +             mutex_unlock(&instance->cdev->lock);
>               thermal_cdev_update(cdev);
>       }
>       return 0;
> diff --git a/drivers/thermal/gov_bang_bang.c b/drivers/thermal/gov_bang_bang.c
> index 70836c5..39d1519 100644
> --- a/drivers/thermal/gov_bang_bang.c
> +++ b/drivers/thermal/gov_bang_bang.c
> @@ -65,7 +65,9 @@ static void thermal_zone_trip_update(struct thermal_zone_device *tz, int trip)
>               dev_dbg(&instance->cdev->device, "target=%d\n",
>                                       (int)instance->target);
>
> +             mutex_lock(&instance->cdev->lock);
>               instance->cdev->updated = false; /* cdev needs update */
> +             mutex_unlock(&instance->cdev->lock);
>       }
>
>       mutex_unlock(&tz->lock);
> diff --git a/drivers/thermal/power_allocator.c b/drivers/thermal/power_allocator.c
> index 1246aa6..2cc5616 100644
> --- a/drivers/thermal/power_allocator.c
> +++ b/drivers/thermal/power_allocator.c
> @@ -529,7 +529,9 @@ static void allow_maximum_power(struct thermal_zone_device *tz)
>                       continue;
>
>               instance->target = 0;
> +             mutex_lock(&instance->cdev->lock);
>               instance->cdev->updated = false;
> +             mutex_unlock(&instance->cdev->lock);
>               thermal_cdev_update(instance->cdev);
>       }
>   }
> diff --git a/drivers/thermal/step_wise.c b/drivers/thermal/step_wise.c
> index ea9366a..bcef2e7 100644
> --- a/drivers/thermal/step_wise.c
> +++ b/drivers/thermal/step_wise.c
> @@ -175,7 +175,9 @@ static void thermal_zone_trip_update(struct thermal_zone_device *tz, int trip)
>                       update_passive_instance(tz, trip_type, -1);
>
>               instance->initialized = true;
> +             mutex_lock(&instance->cdev->lock);
>               instance->cdev->updated = false; /* cdev needs update */
> +             mutex_unlock(&instance->cdev->lock);
>       }
>
>       mutex_unlock(&tz->lock);
> diff --git a/drivers/thermal/thermal_core.c b/drivers/thermal/thermal_core.c
> index a0a8fd1..9d23109 100644
> --- a/drivers/thermal/thermal_core.c
> +++ b/drivers/thermal/thermal_core.c
> @@ -1089,7 +1089,9 @@ int power_actor_set_power(struct thermal_cooling_device *cdev,
>               return ret;
>
>       instance->target = state;
> +     mutex_lock(&cdev->lock);
>       cdev->updated = false;
> +     mutex_unlock(&cdev->lock);
>       thermal_cdev_update(cdev);
>
>       return 0;
> @@ -1619,11 +1621,13 @@ void thermal_cdev_update(struct thermal_cooling_device *cdev)
>       struct thermal_instance *instance;
>       unsigned long target = 0;
>
> +     mutex_lock(&cdev->lock);
>       /* cooling device is updated*/
> -     if (cdev->updated)
> +     if (cdev->updated) {
> +             mutex_unlock(&cdev->lock);
>               return;
> +     }
>
> -     mutex_lock(&cdev->lock);
>       /* Make sure cdev enters the deepest cooling state */
>       list_for_each_entry(instance, &cdev->thermal_instances, cdev_node) {
>               dev_dbg(&cdev->device, "zone%d->target=%lu\n",
> @@ -1633,9 +1637,9 @@ void thermal_cdev_update(struct thermal_cooling_device *cdev)
>               if (instance->target > target)
>                       target = instance->target;
>       }
> -     mutex_unlock(&cdev->lock);
>       cdev->ops->set_cur_state(cdev, target);
>       cdev->updated = true;
> +     mutex_unlock(&cdev->lock);
>       trace_cdev_update(cdev, target);
>       dev_dbg(&cdev->device, "set to state %lu\n", target);
>   }

If there are no comments/issues, could you please consider this for next
merge window?

Thanks,
Michele
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