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

[Michele DiGiorgio <michele.digiorgio@arm.com>]

Hi Eduardo, Rui,

On 06/06/16 13:51, Javi Merino wrote:
> On Thu, Jun 02, 2016 at 03:25:31PM +0100, 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@intel.com>
>> CC: Eduardo Valentin <edubezval@gmail.com>
>> CC: Peter Feuerer <peter@piie.net>
>> Reported-by: Toby Huang <toby.huang@arm.com>
>> Signed-off-by: Michele Di Giorgio <michele.digiorgio@arm.com>
>> ---
>> 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().
>
> True.  In any case, the mutex needs to cover set_cur_state() in
> thermal_cdev_update().  This fixes the race condition, so I'm happy
> with it.
>
> Reviewed-by: Javi Merino <javi.merino@arm.com>
>
>>   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(-)
>

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

Thanks,
Michele
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