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

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Message-ID: <20210625133425.GX22278@shell.armlinux.org.uk>
Date:   Fri, 25 Jun 2021 14:34:25 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Petr Mladek <pmladek@...e.com>
Cc:     John Ogness <john.ogness@...utronix.de>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        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 Fri, Jun 25, 2021 at 02:36:23PM +0200, Petr Mladek wrote:
> 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.

As I seem to recall, the guy in ARM Ltd who was working on this seemed
to drift away and it never got finished - however, I've always carried
platform specific hacks in my tree to make this work from FIQ on the
platforms I cared about:

http://git.armlinux.org.uk/cgit/linux-arm.git/commit/?h=fiq

Not suitable for mainline like that. I'm not aware of anyone working on
it now.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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