linux-kernel - Re: [PATCH v2 1/7] entry: add arch_in_rcu

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Date:   Fri, 21 Jan 2022 18:34:55 +0100
From:   Nicolas Saenz Julienne <nsaenzju@...hat.com>
To:     Mark Rutland <mark.rutland@....com>, linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     aleksandar.qemu.devel@...il.com, alexandru.elisei@....com,
        anup.patel@....com, aou@...s.berkeley.edu, atish.patra@....com,
        borntraeger@...ux.ibm.com, bp@...en8.de, catalin.marinas@....com,
        chenhuacai@...nel.org, dave.hansen@...ux.intel.com,
        frankja@...ux.ibm.com, frederic@...nel.org, gor@...ux.ibm.com,
        hca@...ux.ibm.com, james.morse@....com, jmattson@...gle.com,
        joro@...tes.org, maz@...nel.org, mingo@...hat.com,
        mpe@...erman.id.au, palmer@...belt.com, paulmck@...nel.org,
        paul.walmsley@...ive.com, pbonzini@...hat.com, seanjc@...gle.com,
        suzuki.poulose@....com, svens@...ux.ibm.com,
        tsbogend@...ha.franken.de, vkuznets@...hat.com,
        wanpengli@...cent.com, will@...nel.org
Subject: Re: [PATCH v2 1/7] entry: add arch_in_rcu_eqs()

On Wed, 2022-01-19 at 10:58 +0000, Mark Rutland wrote:
> 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@....com>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Christian Borntraeger <borntraeger@...ux.ibm.com>
> Cc: Heiko Carstens <hca@...ux.ibm.com>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Paul E. McKenney <paulmck@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Sven Schnelle <svens@...ux.ibm.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> ---

Reviewed-by: Nicolas Saenz Julienne <nsaenzju@...hat.com>

Thanks,

-- 
Nicolás Sáenz