linux-kernel - Re: [PATCH v3] ring-buffer: Remove 32bit timestamp logic

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Message-ID: <CAHk-=wiKooX5vOu6TgGPEwdX--k0DyE4ntJDU4QzbVFMWGVXFw@mail.gmail.com>
Date: Thu, 14 Dec 2023 11:46:55 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: LKML <linux-kernel@...r.kernel.org>, 
	Linux Trace Kernel <linux-trace-kernel@...r.kernel.org>, Masami Hiramatsu <mhiramat@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 
	Linux Arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH v3] ring-buffer: Remove 32bit timestamp logic

On Thu, 14 Dec 2023 at 09:53, Steven Rostedt <rostedt@...dmis.org> wrote:
>
> +       /*
> +        * For architectures that can not do cmpxchg() in NMI, or require
> +        * disabling interrupts to do 64-bit cmpxchg(), do not allow them
> +        * to record in NMI context.
> +        */
> +       if ((!IS_ENABLED(CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG) ||
> +            (IS_ENABLED(CONFIG_X86_32) && !IS_ENABLED(CONFIG_X86_CMPXCHG64))) &&
> +           unlikely(in_nmi())) {
> +               return NULL;
> +       }

Again, this is COMPLETE GARBAGE.

You're using "ARCH_HAVE_NMI_SAFE_CMPXCHG" to test something that just
isn't what it's about.

Having a NMI-safe cmpxchg does *not* mean that you actualyl have a
NMI-safe 64-bit version.

You can't test it that way.

Stop making random changes that just happen to work on the one machine
you tested it on.

           Linus