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

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Message-ID: <CAHk-=wh5XgB4Jb9cRLe6gh_C_wXK3YevqCLi1BFRk5z1pJDkQA@mail.gmail.com>
Date: Thu, 14 Dec 2023 12:30:10 -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 12:18, Steven Rostedt <rostedt@...dmis.org> wrote:
>
> For this issue of the 64bit cmpxchg, is there any config that works for any
> arch that do not have a safe 64-bit cmpxchg? At least for 486, is the
> second half of the if condition reasonable?
>
>         if (IS_ENABLED(CONFIG_X86_32) && !IS_ENABLED(CONFIG_X86_CMPXCHG64)) {
>                 if (unlikely(in_nmi()))
>                         return NULL;
>         }

No.

Read my email. Don't do random x86-centric things. We have that

  #ifndef system_has_cmpxchg64
      #define system_has_cmpxchg64() false
  #endif

which should work.

NOTE! The above is for 32-bit architectures only! For 64-bit ones
either just use cmpxchg directly. And if you need a 128-bit one,
there's system_has_cmpxchg128...

                 Linus