Re: [PATCH] ARM: imx6q: Fixup RCU usage for cpuidle

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Sep 30, 2020 at 01:20:23PM +0200, Ulf Hansson wrote:
> The commit eb1f00237aca ("lockdep,trace: Expose tracepoints"), started to
> expose us for tracepoints. For imx6q cpuidle, this leads to an RCU splat
> according to below.
> 
> [6.870684] [<c0db7690>] (_raw_spin_lock) from [<c011f6a4>] (imx6q_enter_wait+0x18/0x9c)
> [6.878846] [<c011f6a4>] (imx6q_enter_wait) from [<c09abfb0>] (cpuidle_enter_state+0x168/0x5e4)
> 
> To fix the problem, let's assign the corresponding idlestate->flags the
> CPUIDLE_FLAG_RCU_IDLE bit, which enables us to call rcu_idle_enter|exit()
> at the proper point.
> 
> Reported-by: Dong Aisheng <aisheng.dong@nxp.com>
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Yep, that looks about right.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

> ---
>  arch/arm/mach-imx/cpuidle-imx6q.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm/mach-imx/cpuidle-imx6q.c b/arch/arm/mach-imx/cpuidle-imx6q.c
> index 24dd5bbe60e4..094337dc1bc7 100644
> --- a/arch/arm/mach-imx/cpuidle-imx6q.c
> +++ b/arch/arm/mach-imx/cpuidle-imx6q.c
> @@ -24,7 +24,9 @@ static int imx6q_enter_wait(struct cpuidle_device *dev,
>  		imx6_set_lpm(WAIT_UNCLOCKED);
>  	raw_spin_unlock(&cpuidle_lock);
>  
> +	rcu_idle_enter();
>  	cpu_do_idle();
> +	rcu_idle_exit();
>  
>  	raw_spin_lock(&cpuidle_lock);
>  	if (num_idle_cpus-- == num_online_cpus())
> @@ -44,7 +46,7 @@ static struct cpuidle_driver imx6q_cpuidle_driver = {
>  		{
>  			.exit_latency = 50,
>  			.target_residency = 75,
> -			.flags = CPUIDLE_FLAG_TIMER_STOP,
> +			.flags = CPUIDLE_FLAG_TIMER_STOP | CPUIDLE_FLAG_RCU_IDLE,
>  			.enter = imx6q_enter_wait,
>  			.name = "WAIT",
>  			.desc = "Clock off",
> -- 
> 2.25.1
>