linux-kernel - Re: [PATCH 1/2] context_tracking: Provide helper to determine if we're in IRQ

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Message-ID: <aEmt4Aa3-gULNtic@localhost.localdomain>
Date: Wed, 11 Jun 2025 18:25:04 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Joel Fernandes <joelagnelf@...dia.com>
Cc: linux-kernel@...r.kernel.org, "Paul E. McKenney" <paulmck@...nel.org>,
	Xiongfeng Wang <wangxiongfeng2@...wei.com>, rcu@...r.kernel.org,
	bpf@...r.kernel.org
Subject: Re: [PATCH 1/2] context_tracking: Provide helper to determine if
 we're in IRQ

Le Mon, Jun 09, 2025 at 02:01:23PM -0400, Joel Fernandes a écrit :
> context_tracking keeps track of whether we're handling IRQ well after
> the preempt masks give take it off their books. We need this
> functionality in a follow-up patch to fix a bug. Provide a helper API
> for the same.
> 
> Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>
> ---
>  include/linux/context_tracking_irq.h |  2 ++
>  kernel/context_tracking.c            | 12 ++++++++++++
>  2 files changed, 14 insertions(+)
> 
> diff --git a/include/linux/context_tracking_irq.h b/include/linux/context_tracking_irq.h
> index 197916ee91a4..35a5ad971514 100644
> --- a/include/linux/context_tracking_irq.h
> +++ b/include/linux/context_tracking_irq.h
> @@ -9,6 +9,7 @@ void ct_irq_enter_irqson(void);
>  void ct_irq_exit_irqson(void);
>  void ct_nmi_enter(void);
>  void ct_nmi_exit(void);
> +bool ct_in_irq(void);
>  #else
>  static __always_inline void ct_irq_enter(void) { }
>  static __always_inline void ct_irq_exit(void) { }
> @@ -16,6 +17,7 @@ static inline void ct_irq_enter_irqson(void) { }
>  static inline void ct_irq_exit_irqson(void) { }
>  static __always_inline void ct_nmi_enter(void) { }
>  static __always_inline void ct_nmi_exit(void) { }
> +static inline bool ct_in_irq(void) { return false; }
>  #endif
>  
>  #endif
> diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
> index fb5be6e9b423..d0759ef9a6bd 100644
> --- a/kernel/context_tracking.c
> +++ b/kernel/context_tracking.c
> @@ -392,6 +392,18 @@ noinstr void ct_irq_exit(void)
>  	ct_nmi_exit();
>  }
>  
> +/**
> + * ct_in_irq - check if CPU is in a context-tracked IRQ context.
> + *
> + * Returns true if ct_irq_enter() has been called and ct_irq_exit()
> + * has not yet been called. This indicates the CPU is currently
> + * processing an interrupt.
> + */
> +bool ct_in_irq(void)
> +{
> +	return ct_nmi_nesting() != 0;

If rcu_is_watching() and not in an interrupt, ct_nmi_nesting()
is actually CT_NESTING_IRQ_NONIDLE. If rcu_is_watching() and
in an interrupt, ct_nmi_nesting() can be CT_NESTING_IRQ_NONIDLE + whatever.

So this doesn't work. I wish we could remove that CT_NESTING_IRQ_NONIDLE
that is there for hysterical raisins but that doesn't fit in an urgent pile.

So probably:

bool ct_in_irq(void)
{
	long nesting = ct_nmi_nesting();

	return (nesting && nesting != CT_NESTING_IRQ_NONIDLE);
}

Thanks.

-- 
Frederic Weisbecker
SUSE Labs