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Date: Mon, 6 Jun 2016 13:51:14 +0100
From: Javi Merino <javi.merino@....com>
To: Michele Di Giorgio <michele.digiorgio@....com>
Cc: linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
Zhang Rui <rui.zhang@...el.com>,
Eduardo Valentin <edubezval@...il.com>,
Peter Feuerer <peter@...e.net>
Subject: Re: [PATCH] thermal: fix race condition when updating cooling device
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@...el.com>
> CC: Eduardo Valentin <edubezval@...il.com>
> CC: Peter Feuerer <peter@...e.net>
> Reported-by: Toby Huang <toby.huang@....com>
> Signed-off-by: Michele Di Giorgio <michele.digiorgio@....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@....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(-)
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