linux-kernel - Re: [PATCH 2/2] context_tracking: Invoke Tasks-RCU enter/exit for NMI context

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Message-ID: <ZryncuVGyxL1-OX1@localhost.localdomain>
Date: Wed, 14 Aug 2024 14:47:46 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: neeraj.upadhyay@...nel.org
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, paulmck@...nel.org,
	joel@...lfernandes.org, boqun.feng@...il.com, urezki@...il.com,
	rostedt@...dmis.org, mathieu.desnoyers@...icios.com,
	jiangshanlai@...il.com, qiang.zhang1211@...il.com,
	neeraj.iitr10@...il.com
Subject: Re: [PATCH 2/2] context_tracking: Invoke Tasks-RCU enter/exit for
 NMI context

Le Wed, Aug 07, 2024 at 10:05:06PM +0530, neeraj.upadhyay@...nel.org a écrit :
> From: Neeraj Upadhyay <neeraj.upadhyay@...nel.org>
> 
> rcu_task_enter() and rcu_task_exit() are not called on NMI
> entry and exit. So, Tasks-RCU-Rude grace period wait is required to
> ensure that NMI handlers have entered/exited into Tasks-RCU eqs.
> For architectures which do not require Tasks-RCU-Rude (as the code
> sections where RCU is not watching are marked as noinstr), when
> those architectures switch to not using Tasks-RCU-Rude, NMI handlers
> task exit to eqs will need to be handled correctly for Tasks-RCU holdout
> tasks running on nohz_full CPUs. As it is safe to call these two
> functions from NMI context, remove the in_nmi() check to ensure that
> Tasks-RCU entry/exit is marked correctly for NMI handlers.
> 
> Reported-by: Frederic Weisbecker <frederic@...nel.org>
> Suggested-by: Frederic Weisbecker <frederic@...nel.org>
> Suggested-by: "Paul E. McKenney" <paulmck@...nel.org>
> Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@...nel.org>
> ---
>  kernel/context_tracking.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
> index 152b485a62db..626dd7a173a5 100644
> --- a/kernel/context_tracking.c
> +++ b/kernel/context_tracking.c
> @@ -239,8 +239,7 @@ void noinstr ct_nmi_exit(void)
>  	ct_kernel_exit_state(CT_RCU_WATCHING);
>  	// ... but is no longer watching here.
>  
> -	if (!in_nmi())
> -		rcu_task_exit();
> +	rcu_task_exit();

And now since it is called by all callers of ct_kernel_exit_state(), you
can call it from it directly.

>  }
>  
>  /**
> @@ -273,8 +272,7 @@ void noinstr ct_nmi_enter(void)
>  	 */
>  	if (!rcu_is_watching_curr_cpu()) {
>  
> -		if (!in_nmi())
> -			rcu_task_enter();
> +		rcu_task_enter();
>  
>  		// RCU is not watching here ...
>  		ct_kernel_enter_state(CT_RCU_WATCHING);

Ditto for ct_kernel_enter_state().

Thanks.

> -- 
> 2.40.1
>