linux-kernel - Re: [PATCH RFC v2 rcu 6/8] arch/arm64: Add ARCH_HAS_NMI_SAFE_THIS_CPU

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Message-ID: <Yz1mneessxDSN8V7@FVFF77S0Q05N>
Date:   Wed, 5 Oct 2022 12:12:29 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, rostedt@...dmis.org,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        John Ogness <john.ogness@...utronix.de>,
        Petr Mladek <pmladek@...e.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH RFC v2 rcu 6/8] arch/arm64: Add
 ARCH_HAS_NMI_SAFE_THIS_CPU_OPS Kconfig option

Hi Paul,

On Thu, Sep 29, 2022 at 11:07:29AM -0700, Paul E. McKenney wrote:
> The arm64 architecture uses either an LL/SC loop (old systems) or an LSE
> stadd instruction (new systems) to implement this_cpu_add(), both of which
> are NMI safe.

IIUC "NMI safe" here just means atomic w.r.t. an NMI being taken and modfying
the same location the atomic was targetting (i.e. just like
ARCH_HAVE_NMI_SAFE_CMPXCHG, which arm64 selects today).

Assuming so:

  Acked-by: Mark Rutland <mark.rutland@....com>

Only this patch went to LAKML, so maybe an earlier patch made that clear, but
I didn't spot it.

As one minor nit, it would be nice to align the naming with
ARCH_HAVE_NMI_SAFE_CMPXCHG and select them next to each other in the Kconfig
file is possible, but the Ack stands regardless.

Thanks,
Mark.

> This means that the old and more-efficient srcu_read_lock()
> may be used in NMI context, without the need for srcu_read_lock_nmisafe().
> Therefore, add the new Kconfig option ARCH_HAS_NMI_SAFE_THIS_CPU_OPS to
> arch/arm64/Kconfig, which will cause NEED_SRCU_NMI_SAFE to be deselected,
> thus preserving the current srcu_read_lock() behavior.
> 
> Link: https://lore.kernel.org/all/20220910221947.171557773@linutronix.de/
> 
> Suggested-by: Neeraj Upadhyay <quic_neeraju@...cinc.com>
> Suggested-by: Frederic Weisbecker <frederic@...nel.org>
> Suggested-by: Boqun Feng <boqun.feng@...il.com>
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: John Ogness <john.ogness@...utronix.de>
> Cc: Petr Mladek <pmladek@...e.com>
> Cc: <linux-arm-kernel@...ts.infradead.org>
> ---
>  arch/arm64/Kconfig | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 571cc234d0b3..664725a0b5dd 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -31,6 +31,7 @@ config ARM64
>  	select ARCH_HAS_KCOV
>  	select ARCH_HAS_KEEPINITRD
>  	select ARCH_HAS_MEMBARRIER_SYNC_CORE
> +	select ARCH_HAS_NMI_SAFE_THIS_CPU_OPS
>  	select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
>  	select ARCH_HAS_PTE_DEVMAP
>  	select ARCH_HAS_PTE_SPECIAL
> -- 
> 2.31.1.189.g2e36527f23
>