Re: [PATCH 1/2] entry: Add arch_in_rcu_eqs()

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jul 08, 2025 at 07:27:41PM +1000, Andrew Donnellan wrote:
> From: Mark Rutland <mark.rutland@arm.com>
> 
> All architectures have an interruptible RCU extended quiescent state
> (EQS) as part of their idle sequences, where interrupts can occur
> without RCU watching. Entry code must account for this and wake RCU as
> necessary; the common entry code deals with this in irqentry_enter() by
> treating any interrupt from an idle thread as potentially having
> occurred within an EQS and waking RCU for the duration of the interrupt
> via rcu_irq_enter() .. rcu_irq_exit().
> 
> Some architectures may have other interruptible EQSs which require
> similar treatment. For example, on s390 it is necessary to enable
> interrupts around guest entry in the middle of a period where core KVM
> code has entered an EQS.
> 
> So that architectures can wake RCU in these cases, this patch adds a
> new arch_in_rcu_eqs() hook to the common entry code which is checked in
> addition to the existing is_idle_thread() check, with RCU woken if
> either returns true. A default implementation is provided which always
> returns false, which suffices for most architectures.
> 
> As no architectures currently implement arch_in_rcu_eqs(), there should
> be no functional change as a result of this patch alone. A subsequent
> patch will add an s390 implementation to fix a latent bug with missing
> RCU wakeups.
> 
> [ajd@linux.ibm.com: rebase, fix commit message]
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
> Cc: Heiko Carstens <hca@linux.ibm.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Paul E. McKenney <paulmck@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Sven Schnelle <svens@linux.ibm.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
> Cc: Vasily Gorbik <gor@linux.ibm.com>
> Cc: Alexander Gordeev <agordeev@linux.ibm.com>
> Cc: Janosch Frank <frankja@linux.ibm.com>
> Reviewed-by: Christian Borntraeger <borntraeger@linux.ibm.com>
> Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>

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

> ---
>  include/linux/entry-common.h | 16 ++++++++++++++++
>  kernel/entry/common.c        |  3 ++-
>  2 files changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
> index f94f3fdf15fc..3bf99cbad8a3 100644
> --- a/include/linux/entry-common.h
> +++ b/include/linux/entry-common.h
> @@ -86,6 +86,22 @@ static __always_inline void arch_enter_from_user_mode(struct pt_regs *regs);
>  static __always_inline void arch_enter_from_user_mode(struct pt_regs *regs) {}
>  #endif
>  
> +/**
> + * arch_in_rcu_eqs - Architecture specific check for RCU extended quiescent
> + * states.
> + *
> + * Returns: true if the CPU is potentially in an RCU EQS, false otherwise.
> + *
> + * Architectures only need to define this if threads other than the idle thread
> + * may have an interruptible EQS. This does not need to handle idle threads. It
> + * is safe to over-estimate at the cost of redundant RCU management work.
> + *
> + * Invoked from irqentry_enter()
> + */
> +#ifndef arch_in_rcu_eqs
> +static __always_inline bool arch_in_rcu_eqs(void) { return false; }
> +#endif
> +
>  /**
>   * enter_from_user_mode - Establish state when coming from user mode
>   *
> diff --git a/kernel/entry/common.c b/kernel/entry/common.c
> index a8dd1f27417c..eb52d38e8099 100644
> --- a/kernel/entry/common.c
> +++ b/kernel/entry/common.c
> @@ -220,7 +220,8 @@ noinstr irqentry_state_t irqentry_enter(struct pt_regs *regs)
>  	 * TINY_RCU does not support EQS, so let the compiler eliminate
>  	 * this part when enabled.
>  	 */
> -	if (!IS_ENABLED(CONFIG_TINY_RCU) && is_idle_task(current)) {
> +	if (!IS_ENABLED(CONFIG_TINY_RCU) &&
> +	    (is_idle_task(current) || arch_in_rcu_eqs())) {
>  		/*
>  		 * If RCU is not watching then the same careful
>  		 * sequence vs. lockdep and tracing is required
> -- 
> 2.50.0
>