• Michele Di Giorgio's avatar
    thermal: fix race condition when updating cooling device · d0b7306d
    Michele Di Giorgio authored
    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;                     ||
    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: default avatarToby Huang <toby.huang@arm.com>
    Signed-off-by: default avatarMichele Di Giorgio <michele.digiorgio@arm.com>
    Reviewed-by: default avatarJavi Merino <javi.merino@arm.com>
    Signed-off-by: default avatarZhang Rui <rui.zhang@intel.com>
gov_bang_bang.c 3.88 KB