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Date:   Wed, 19 Jan 2022 18:35:42 +0100
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Subject: Re: [PATCH v2 1/7] entry: add arch_in_rcu_eqs()
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To:     Mark Rutland <mark.rutland@arm.com>, linux-kernel@vger.kernel.org,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>
Cc:     aleksandar.qemu.devel@gmail.com, alexandru.elisei@arm.com,
        anup.patel@wdc.com, aou@eecs.berkeley.edu, atish.patra@wdc.com,
        bp@alien8.de, catalin.marinas@arm.com, chenhuacai@kernel.org,
        dave.hansen@linux.intel.com, frankja@linux.ibm.com,
        frederic@kernel.org, gor@linux.ibm.com, hca@linux.ibm.com,
        james.morse@arm.com, jmattson@google.com, joro@8bytes.org,
        maz@kernel.org, mingo@redhat.com, mpe@ellerman.id.au,
        nsaenzju@redhat.com, palmer@dabbelt.com, paulmck@kernel.org,
        paul.walmsley@sifive.com, pbonzini@redhat.com, seanjc@google.com,
        suzuki.poulose@arm.com, svens@linux.ibm.com,
        tsbogend@alpha.franken.de, vkuznets@redhat.com,
        wanpengli@tencent.com, will@kernel.org
References: <20220119105854.3160683-1-mark.rutland@arm.com>
 <20220119105854.3160683-2-mark.rutland@arm.com>
From:   Christian Borntraeger <borntraeger@linux.ibm.com>
In-Reply-To: <20220119105854.3160683-2-mark.rutland@arm.com>
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Am 19.01.22 um 11:58 schrieb Mark Rutland:
> 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 with 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 is it necessary to enable
> interrupts around guest entry in the middle of a period where core KVM
> code has entered an EQS.
> 
> So that architectueres 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.
> 
> 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>

Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
> ---
>   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 2e2b8d6140ed4..f1b91a13a15a6 100644
> --- a/include/linux/entry-common.h
> +++ b/include/linux/entry-common.h
> @@ -99,6 +99,22 @@ static inline __must_check int arch_syscall_enter_tracehook(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 d5a61d565ad5d..b13d4e0b0b643 100644
> --- a/kernel/entry/common.c
> +++ b/kernel/entry/common.c
> @@ -349,7 +349,8 @@ 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