linux-kernel - Re: [PATCH v3 1/2] nmi_backtrace: Allow excluding an arbitrary CPU

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Date:   Fri, 4 Aug 2023 09:06:07 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Petr Mladek <pmladek@...e.com>, Arnd Bergmann <arnd@...db.de>,
        Borislav Petkov <bp@...en8.de>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "David S. Miller" <davem@...emloft.net>,
        Gaosheng Cui <cuigaosheng1@...wei.com>,
        "Gautham R. Shenoy" <gautham.shenoy@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
        Guo Ren <guoren@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Huacai Chen <chenhuacai@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Jianmin Lv <lvjianmin@...ngson.cn>,
        Jinyang He <hejinyang@...ngson.cn>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Lecopzer Chen <lecopzer.chen@...iatek.com>,
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        Peter Zijlstra <peterz@...radead.org>,
        Pingfan Liu <kernelfans@...il.com>,
        Qing Zhang <zhangqing@...ngson.cn>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Russell King <linux@...linux.org.uk>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tom Rix <trix@...hat.com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Valentin Schneider <vschneid@...hat.com>,
        WANG Xuerui <kernel@...0n.name>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        loongarch@...ts.linux.dev, sparclinux@...r.kernel.org,
        x86@...nel.org
Subject: Re: [PATCH v3 1/2] nmi_backtrace: Allow excluding an arbitrary CPU

Hi,

On Fri, Aug 4, 2023 at 8:02 AM Michal Hocko <mhocko@...e.com> wrote:
>
> > > It would have been slightly safer to modify arch_trigger_cpumask_backtrace
> > > by switching arguments so that some leftovers are captured easier.
> >
> > I'm not sure I understand. Oh, you're saying make the prototype of
> > arch_trigger_cpumask_backtrace() incompatible so that if someone is
> > directly calling it then it'll be a compile-time error?
>
> exactly. bool to int promotion would be too easy to miss while the
> pointer to int would complain loudly.
>
> > I guess the
> > hope is that nobody is calling that directly and they're calling
> > through the trigger_...() functions.
>
> Hope is one thing, being preventive another.
>
> > For now I'm going to leave this alone.
>
> If you are going to send another version then please consider this. Not
> a hard requirement but better.

If I do send another version, do you have any suggestions for how to
change this to make it incompatible? I guess swapping the order of the
parameters would be best? I considered doing that for v4 but I felt
like long term the current order of the parameters was better. I also
considered a rename, but that different problems. ;-) If I rename both
the #define and the function then if someone has an out-of-tree patch
adding arch_trigger_cpumask_backtrace() for another architecture, like
say arm64, then there would be no compile-time failure indicating that
the out-of-tree patch needs updating. I could rename the functions but
_not_ the #define, I guess?