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

From: Doug Anderson <dianders@chromium.org>
To: Michal Hocko <mhocko@suse.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Petr Mladek <pmladek@suse.com>,  Arnd Bergmann <arnd@arndb.de>,
	Borislav Petkov <bp@alien8.de>,
	 Christophe Leroy <christophe.leroy@csgroup.eu>,
	"Darrick J. Wong" <djwong@kernel.org>,
	 Dave Hansen <dave.hansen@linux.intel.com>,
	"David S. Miller" <davem@davemloft.net>,
	 Gaosheng Cui <cuigaosheng1@huawei.com>,
	"Gautham R. Shenoy" <gautham.shenoy@amd.com>,
	 Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Guilherme G. Piccoli" <gpiccoli@igalia.com>,
	 Guo Ren <guoren@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
	Huacai Chen <chenhuacai@kernel.org>,
	 Ingo Molnar <mingo@redhat.com>,
	"Jason A. Donenfeld" <Jason@zx2c4.com>,
	Jianmin Lv <lvjianmin@loongson.cn>,
	 Jinyang He <hejinyang@loongson.cn>,
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	 Kees Cook <keescook@chromium.org>,
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	Valentin Schneider <vschneid@redhat.com>,
	 WANG Xuerui <kernel@xen0n.name>,
	linux-arm-kernel@lists.infradead.org,
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Subject: Re: [PATCH v3 1/2] nmi_backtrace: Allow excluding an arbitrary CPU
Date: Fri, 4 Aug 2023 09:06:07 -0700	[thread overview]
Message-ID: <CAD=FV=XhksUsBKjkD08YxdmxvvbgoZH-va7WnPptkYA6EOpPsg@mail.gmail.com> (raw)
In-Reply-To: <ZM0S9gKBBiu83kFq@dhcp22.suse.cz>

Hi,

On Fri, Aug 4, 2023 at 8:02 AM Michal Hocko <mhocko@suse.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?