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

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Date:   Tue, 25 Feb 2020 02:30:14 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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>,
        Marc Zyngier <maz@...nel.org>
Subject: Re: [PATCH v4 02/27] hardirq/nmi: Allow nested nmi_enter()

On Mon, Feb 24, 2020 at 01:13:31PM +0100, Petr Mladek wrote:
> On Fri 2020-02-21 23:21:30, Frederic Weisbecker wrote:
> > 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?
> 
> Great catch!
> 
> There is plenty of space in the printk_context variable. I would
> reserve one byte there for the NMI context to be on the safe side
> and be done with it.
> 
> It should never overflow. The BUG_ON(in_nmi() == NMI_MASK)
> in nmi_enter() will trigger much earlier.
> 
> Also I hope that printk_context will get removed with
> the lockless printk() implementation soon anyway.

Cool, because it's sad that we have to mimic/mirror the preempt count
with printk_context just for the sake of debugging preempt_count_add()
and other early code in nmi_enter().

The diff below looks good, thanks!

Reviewed-by: Frederic Weisbecker <frederic@...nel.org>


> 
> 
> diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
> index c8e6ab689d42..109c5ab70a0c 100644
> --- a/kernel/printk/internal.h
> +++ b/kernel/printk/internal.h
> @@ -6,9 +6,11 @@
>  
>  #ifdef CONFIG_PRINTK
>  
> -#define PRINTK_SAFE_CONTEXT_MASK	 0x3fffffff
> -#define PRINTK_NMI_DIRECT_CONTEXT_MASK	 0x40000000
> -#define PRINTK_NMI_CONTEXT_MASK		 0x80000000
> +#define PRINTK_SAFE_CONTEXT_MASK	0x007ffffff
> +#define PRINTK_NMI_DIRECT_CONTEXT_MASK	0x008000000
> +#define PRINTK_NMI_CONTEXT_MASK		0xff0000000
> +
> +#define PRINTK_NMI_CONTEXT_OFFSET	0x010000000
>  
>  extern raw_spinlock_t logbuf_lock;
>  
> diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
> index b4045e782743..e8989418a139 100644
> --- a/kernel/printk/printk_safe.c
> +++ b/kernel/printk/printk_safe.c
> @@ -296,12 +296,12 @@ static __printf(1, 0) int vprintk_nmi(const char *fmt, va_list args)
>  
>  void notrace printk_nmi_enter(void)
>  {
> -	this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK);
> +	this_cpu_add(printk_context, PRINTK_NMI_CONTEXT_OFFSET);
>  }
>  
>  void notrace printk_nmi_exit(void)
>  {
> -	this_cpu_and(printk_context, ~PRINTK_NMI_CONTEXT_MASK);
> +	this_cpu_sub(printk_context, PRINTK_NMI_CONTEXT_OFFSET);
>  }
>  
>  /*
> 
> Best Regards,
> Petr