Re: [PATCH] cpu_pm: replace raw_notifier to atomic_notifier

[Alex Shi]

It is a serious bug: add a waiting lock in idle and cause boot failure
in arm/arm64 RT.

Any more comments for this change?

On 07/06/2017 04:47 PM, Alex Shi wrote:
> This patch replace a rwlock and raw notifier by atomic notifier which
> protected by spin_lock and rcu.
> 
> The first to reason to have this replace is due to a 'scheduling while
>  atomic' bug of RT kernel on arm/arm64 platform. On arm/arm64, rwlock
> cpu_pm_notifier_lock in cpu_pm cause a potential schedule after irq
> disable in idle call chain:
> 
> cpu_startup_entry
>   cpu_idle_loop
>     local_irq_disable()
>     cpuidle_idle_call
>       call_cpuidle
>         cpuidle_enter
>           cpuidle_enter_state
>             ->enter :arm_enter_idle_state
>               cpu_pm_enter/exit
>                 CPU_PM_CPU_IDLE_ENTER
>                   read_lock(&cpu_pm_notifier_lock); <-- sleep in idle
>                      __rt_spin_lock();
>                         schedule();
> 
> The kernel panic is here:
> [    4.609601] BUG: scheduling while atomic: swapper/1/0/0x00000002
> [    4.609608] [<ffff0000086fae70>] arm_enter_idle_state+0x18/0x70
> [    4.609614] Modules linked in:
> [    4.609615] [<ffff0000086f9298>] cpuidle_enter_state+0xf0/0x218
> [    4.609620] [<ffff0000086f93f8>] cpuidle_enter+0x18/0x20
> [    4.609626] Preemption disabled at:
> [    4.609627] [<ffff0000080fa234>] call_cpuidle+0x24/0x40
> [    4.609635] [<ffff000008882fa4>] schedule_preempt_disabled+0x1c/0x28
> [    4.609639] [<ffff0000080fa49c>] cpu_startup_entry+0x154/0x1f8
> [    4.609645] [<ffff00000808e004>] secondary_start_kernel+0x15c/0x1a0
> 
> Daniel Lezcano said this notification is needed on arm/arm64 platforms.
> Sebastian suggested using atomic_notifier instead of rwlock, which is not
> only removing the sleeping in idle, but also getting better latency
> improvement.
> 
> This patch passed Fengguang's 0day testing.
> 
> Signed-off-by: Alex Shi <alex.shi@xxxxxxxxxx>
> Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Anders Roxell <anders.roxell@xxxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> Cc: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
> Cc: linux-rt-users <linux-rt-users@xxxxxxxxxxxxxxx>
> ---
>  kernel/cpu_pm.c | 43 ++++++-------------------------------------
>  1 file changed, 6 insertions(+), 37 deletions(-)
> 
> diff --git a/kernel/cpu_pm.c b/kernel/cpu_pm.c
> index 009cc9a..10f4640 100644
> --- a/kernel/cpu_pm.c
> +++ b/kernel/cpu_pm.c
> @@ -22,14 +22,13 @@
>  #include <linux/spinlock.h>
>  #include <linux/syscore_ops.h>
>  
> -static DEFINE_RWLOCK(cpu_pm_notifier_lock);
> -static RAW_NOTIFIER_HEAD(cpu_pm_notifier_chain);
> +static ATOMIC_NOTIFIER_HEAD(cpu_pm_notifier_chain);
>  
>  static int cpu_pm_notify(enum cpu_pm_event event, int nr_to_call, int *nr_calls)
>  {
>  	int ret;
>  
> -	ret = __raw_notifier_call_chain(&cpu_pm_notifier_chain, event, NULL,
> +	ret = __atomic_notifier_call_chain(&cpu_pm_notifier_chain, event, NULL,
>  		nr_to_call, nr_calls);
>  
>  	return notifier_to_errno(ret);
> @@ -47,14 +46,7 @@ static int cpu_pm_notify(enum cpu_pm_event event, int nr_to_call, int *nr_calls)
>   */
>  int cpu_pm_register_notifier(struct notifier_block *nb)
>  {
> -	unsigned long flags;
> -	int ret;
> -
> -	write_lock_irqsave(&cpu_pm_notifier_lock, flags);
> -	ret = raw_notifier_chain_register(&cpu_pm_notifier_chain, nb);
> -	write_unlock_irqrestore(&cpu_pm_notifier_lock, flags);
> -
> -	return ret;
> +	return atomic_notifier_chain_register(&cpu_pm_notifier_chain, nb);
>  }
>  EXPORT_SYMBOL_GPL(cpu_pm_register_notifier);
>  
> @@ -69,14 +61,7 @@ EXPORT_SYMBOL_GPL(cpu_pm_register_notifier);
>   */
>  int cpu_pm_unregister_notifier(struct notifier_block *nb)
>  {
> -	unsigned long flags;
> -	int ret;
> -
> -	write_lock_irqsave(&cpu_pm_notifier_lock, flags);
> -	ret = raw_notifier_chain_unregister(&cpu_pm_notifier_chain, nb);
> -	write_unlock_irqrestore(&cpu_pm_notifier_lock, flags);
> -
> -	return ret;
> +	return atomic_notifier_chain_unregister(&cpu_pm_notifier_chain, nb);
>  }
>  EXPORT_SYMBOL_GPL(cpu_pm_unregister_notifier);
>  
> @@ -100,7 +85,6 @@ int cpu_pm_enter(void)
>  	int nr_calls;
>  	int ret = 0;
>  
> -	read_lock(&cpu_pm_notifier_lock);
>  	ret = cpu_pm_notify(CPU_PM_ENTER, -1, &nr_calls);
>  	if (ret)
>  		/*
> @@ -108,7 +92,6 @@ int cpu_pm_enter(void)
>  		 * PM entry who are notified earlier to prepare for it.
>  		 */
>  		cpu_pm_notify(CPU_PM_ENTER_FAILED, nr_calls - 1, NULL);
> -	read_unlock(&cpu_pm_notifier_lock);
>  
>  	return ret;
>  }
> @@ -128,13 +111,7 @@ EXPORT_SYMBOL_GPL(cpu_pm_enter);
>   */
>  int cpu_pm_exit(void)
>  {
> -	int ret;
> -
> -	read_lock(&cpu_pm_notifier_lock);
> -	ret = cpu_pm_notify(CPU_PM_EXIT, -1, NULL);
> -	read_unlock(&cpu_pm_notifier_lock);
> -
> -	return ret;
> +	return cpu_pm_notify(CPU_PM_EXIT, -1, NULL);
>  }
>  EXPORT_SYMBOL_GPL(cpu_pm_exit);
>  
> @@ -159,7 +136,6 @@ int cpu_cluster_pm_enter(void)
>  	int nr_calls;
>  	int ret = 0;
>  
> -	read_lock(&cpu_pm_notifier_lock);
>  	ret = cpu_pm_notify(CPU_CLUSTER_PM_ENTER, -1, &nr_calls);
>  	if (ret)
>  		/*
> @@ -167,7 +143,6 @@ int cpu_cluster_pm_enter(void)
>  		 * PM entry who are notified earlier to prepare for it.
>  		 */
>  		cpu_pm_notify(CPU_CLUSTER_PM_ENTER_FAILED, nr_calls - 1, NULL);
> -	read_unlock(&cpu_pm_notifier_lock);
>  
>  	return ret;
>  }
> @@ -190,13 +165,7 @@ EXPORT_SYMBOL_GPL(cpu_cluster_pm_enter);
>   */
>  int cpu_cluster_pm_exit(void)
>  {
> -	int ret;
> -
> -	read_lock(&cpu_pm_notifier_lock);
> -	ret = cpu_pm_notify(CPU_CLUSTER_PM_EXIT, -1, NULL);
> -	read_unlock(&cpu_pm_notifier_lock);
> -
> -	return ret;
> +	return cpu_pm_notify(CPU_CLUSTER_PM_EXIT, -1, NULL);
>  }
>  EXPORT_SYMBOL_GPL(cpu_cluster_pm_exit);
>  
>