* Re: [PATCH RFC v2 rcu 6/8] arch/arm64: Add ARCH_HAS_NMI_SAFE_THIS_CPU_OPS Kconfig option
2022-09-29 18:07 ` [PATCH RFC v2 rcu 6/8] arch/arm64: Add ARCH_HAS_NMI_SAFE_THIS_CPU_OPS Kconfig option Paul E. McKenney
@ 2022-10-05 11:12 ` Mark Rutland
0 siblings, 0 replies; 2+ messages in thread
From: Mark Rutland @ 2022-10-05 11:12 UTC (permalink / raw)
To: Paul E. McKenney
Cc: rcu, linux-kernel, kernel-team, rostedt, Neeraj Upadhyay,
Frederic Weisbecker, Boqun Feng, Catalin Marinas, Will Deacon,
Thomas Gleixner, John Ogness, Petr Mladek, linux-arm-kernel
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@arm.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@quicinc.com>
> Suggested-by: Frederic Weisbecker <frederic@kernel.org>
> Suggested-by: Boqun Feng <boqun.feng@gmail.com>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: John Ogness <john.ogness@linutronix.de>
> Cc: Petr Mladek <pmladek@suse.com>
> Cc: <linux-arm-kernel@lists.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
>
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