linux-kernel - Re: [PATCH printk v3 4/6] printk: remove NMI tracking

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Date:   Fri, 25 Jun 2021 14:36:23 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, Russell King <linux@...linux.org.uk>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Ingo Molnar <mingo@...hat.com>, Marc Zyngier <maz@...nel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Pekka Enberg <penberg@...nel.org>,
        Mike Rapoport <rppt@...nel.org>,
        "Wolfram Sang (Renesas)" <wsa+renesas@...g-engineering.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Nick Terrell <terrelln@...com>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Vlastimil Babka <vbabka@...e.cz>,
        linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH printk v3 4/6] printk: remove NMI tracking

On Thu 2021-06-24 13:17:46, John Ogness wrote:
> All NMI contexts are handled the same as the safe context: store the
> message and defer printing. There is no need to have special NMI
> context tracking for this. Using in_nmi() is enough.
> 
> Signed-off-by: John Ogness <john.ogness@...utronix.de>
> Reviewed-by: Petr Mladek <pmladek@...e.com>
> ---
>  arch/arm/kernel/smp.c       |  2 --
>  arch/powerpc/kexec/crash.c  |  3 ---
>  include/linux/hardirq.h     |  2 --
>  include/linux/printk.h      | 12 ------------
>  init/Kconfig                |  5 -----
>  kernel/printk/internal.h    |  6 ------
>  kernel/printk/printk_safe.c | 37 +------------------------------------
>  kernel/trace/trace.c        |  2 --
>  8 files changed, 1 insertion(+), 68 deletions(-)
> 
> diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
> index 74679240a9d8..0dd2d733ad62 100644
> --- a/arch/arm/kernel/smp.c
> +++ b/arch/arm/kernel/smp.c
> @@ -668,9 +668,7 @@ static void do_handle_IPI(int ipinr)
>  		break;
>  
>  	case IPI_CPU_BACKTRACE:
> -		printk_nmi_enter();
>  		nmi_cpu_backtrace(get_irq_regs());
> -		printk_nmi_exit();

It looks to me that in_nmi() returns false here. As a result,
nmi_cpu_backtrace() might newly call consoles immediately.

If I recall correctly, arm does not have a proper NMI.
And this is just some special case of a "normal" IRQ.

And indeed, nmi_enter() is called only from handle_fiq_as_nmi()
and it is just a boiler plate.

If I am right, we should replace printk_nmi_enter() with
printk_safe_enter_irqsave(flags) or so.

Even better solution might be to call this within
nmi_enter()/nmi_exit(). But I am not sure if this is what
the arm people want.

Best Regards,
Petr

PS: Sigh, I have skipped this patch yesterday because it already had
my Reviewed-by. And I missed it before...