Re: [PATCH v4 02/27] hardirq/nmi: Allow nested nmi_enter()

[Frederic Weisbecker]

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@xxxxxxxxxxxxx>
> Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>
> Acked-by: Will Deacon <will@xxxxxxxxxx>
> Acked-by: Marc Zyngier <maz@xxxxxxxxxx>
> ---
>  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?