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SubjectRe: [PATCH V1 0/1] tick: broadcast-hrtimer: Fix a race in bc_set_next
+linux-renesas-soc

In hope that the issue reported in [1] was seen by other members of
Renesas community.

[1] https://lkml.org/lkml/2019/9/18/711

On Wed, Sep 18, 2019 at 04:41:37PM +0200, Balasubramani Vivekanandan wrote:
> I was investigating a rcu stall warning on ARM64 Renesas Rcar3
> platform. On analysis I found that rcu stall warning was because the
> rcu_preempt kthread was starved of cpu time. rcu_preempt was blocked in
> the function schedule_timeout() and never woken up. On further
> investigation I found that local timer interrupts were not happening on
> the cpu where the rcu_preempt kthread was blocked. So the rcu_preempt
> was not woken up after timeout.
> I continued my analysis to debug why the timer failed on the cpu. I
> found that when cpu goes through idle state cycle, the timer failure
> happens. When the cpu enters the idle state it subscribes to the tick
> broadcast clock and shutsdown the local timer. Then on exit from idle
> state the local timer is programmed to fire interrupts. But I found that
> the during the error scenario, cpu fails to program the local timer on
> exit from idle state. The below code in
> __tick_broadcast_oneshot_control() is where the idle code exit path goes
> through and fails to program the timer hardware
>
> now = ktime_get();
> if (dev->next_event <= now) {
> cpumask_set_cpu(cpu, tick_broadcast_force_mask);
> goto out;
> }
>
> The value in next_event will be earlier than current time because the
> tick broadcast clock did not wake up the cpu on its subcribed
> timeout. Later when the cpu is woken up due to some other event this
> condition will arise. After the cpu woken up, any further timeout
> requests by any task on the cpu might fail to program the timer
> hardware because the value in next_event will be earlier than the
> current time.
> Then I focussed on why the tick broadcast clock failed to wake up the
> cpu. I noticed a race condition in the hrtimer based tick broadcast
> clock. The race condition results in a condition where the tick
> broadcast hrtimer is never restarted. I have created a patch to fix the
> race condition. Please review
>
> Balasubramani Vivekanandan (1):
> tick: broadcast-hrtimer: Fix a race in bc_set_next
>
> kernel/time/tick-broadcast-hrtimer.c | 58 ++++++++++++++++++++++------
> kernel/time/tick-broadcast.c | 2 +
> 2 files changed, 48 insertions(+), 12 deletions(-)
>
> --
> 2.17.1
>

--
Best Regards,
Eugeniu

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