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

From: Petr Mladek <pmladek@suse.com>
To: John Ogness <john.ogness@linutronix.de>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel@vger.kernel.org,
	Russell King <linux@armlinux.org.uk>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>, Ingo Molnar <mingo@redhat.com>,
	Marc Zyngier <maz@kernel.org>,
	Valentin Schneider <valentin.schneider@arm.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Pekka Enberg <penberg@kernel.org>,
	Mike Rapoport <rppt@kernel.org>,
	"Wolfram Sang (Renesas)" <wsa+renesas@sang-engineering.com>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Frederic Weisbecker <frederic@kernel.org>,
	Kees Cook <keescook@chromium.org>,
	Masahiro Yamada <masahiroy@kernel.org>,
	Nathan Chancellor <nathan@kernel.org>,
	Sami Tolvanen <samitolvanen@google.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Nick Terrell <terrelln@fb.com>,
	Chris Wilson <chris@chris-wilson.co.uk>,
	Vlastimil Babka <vbabka@suse.cz>,
	linux-arm-kernel@lists.infradead.org,
	linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH printk v3 4/6] printk: remove NMI tracking
Date: Fri, 25 Jun 2021 14:36:23 +0200	[thread overview]
Message-ID: <YNXNx68CXrI9QpHD@alley> (raw)
In-Reply-To: <20210624111148.5190-5-john.ogness@linutronix.de>

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@linutronix.de>
> Reviewed-by: Petr Mladek <pmladek@suse.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...