Re: [PATCH V1 0/1] tick: broadcast-hrtimer: Fix a race in bc_set_next

[Eugeniu Rosca]

+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