linux-kernel - Re: [PATCH v4 02/27] hardirq/nmi: Allow nested nmi

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Date:   Fri, 21 Feb 2020 23:21:30 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        rostedt@...dmis.org, mingo@...nel.org, joel@...lfernandes.org,
        gregkh@...uxfoundation.org, gustavo@...eddedor.com,
        tglx@...utronix.de, paulmck@...nel.org, josh@...htriplett.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        luto@...nel.org, tony.luck@...el.com, dan.carpenter@...cle.com,
        mhiramat@...nel.org, Will Deacon <will@...nel.org>,
        Petr Mladek <pmladek@...e.com>, Marc Zyngier <maz@...nel.org>
Subject: Re: [PATCH v4 02/27] hardirq/nmi: Allow nested nmi_enter()

On Fri, Feb 21, 2020 at 02:34:18PM +0100, Peter Zijlstra wrote:
> Since there are already a number of sites (ARM64, PowerPC) that
> effectively nest nmi_enter(), lets make the primitive support this
> before adding even more.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Reviewed-by: Petr Mladek <pmladek@...e.com>
> Acked-by: Will Deacon <will@...nel.org>
> Acked-by: Marc Zyngier <maz@...nel.org>
> ---
>  arch/arm64/include/asm/hardirq.h |    4 ++--
>  arch/arm64/kernel/sdei.c         |   14 ++------------
>  arch/arm64/kernel/traps.c        |    8 ++------
>  arch/powerpc/kernel/traps.c      |   22 ++++++----------------
>  include/linux/hardirq.h          |    5 ++++-
>  include/linux/preempt.h          |    4 ++--
>  kernel/printk/printk_safe.c      |    6 ++++--
>  7 files changed, 22 insertions(+), 41 deletions(-)
> 
> --- a/kernel/printk/printk_safe.c
> +++ b/kernel/printk/printk_safe.c
> @@ -296,12 +296,14 @@ static __printf(1, 0) int vprintk_nmi(co
>  
>  void notrace printk_nmi_enter(void)
>  {
> -	this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK);
> +	if (!in_nmi())
> +		this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK);
>  }
>  
>  void notrace printk_nmi_exit(void)
>  {
> -	this_cpu_and(printk_context, ~PRINTK_NMI_CONTEXT_MASK);
> +	if (!in_nmi())
> +		this_cpu_and(printk_context, ~PRINTK_NMI_CONTEXT_MASK);
>  }

If the outermost NMI is interrupted while between printk_nmi_enter()
and preempt_count_add(), there is still a risk that we race and clear?