linux-kernel - Re: [PATCH v10 07/14] unwind_user/deferred: Make unwind deferral requests NMI-safe

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Message-ID: <66B9E72C-4FDF-46DF-9231-BED06A6000D9@goodmis.org>
Date: Thu, 19 Jun 2025 05:10:20 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Peter Zijlstra <peterz@...radead.org>
CC: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
 bpf@...r.kernel.org, x86@...nel.org, Masami Hiramatsu <mhiramat@...nel.org>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Josh Poimboeuf <jpoimboe@...nel.org>, Ingo Molnar <mingo@...nel.org>,
 Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
 Thomas Gleixner <tglx@...utronix.de>, Andrii Nakryiko <andrii@...nel.org>,
 Indu Bhagat <indu.bhagat@...cle.com>, "Jose E. Marchesi" <jemarch@....org>,
 Beau Belgrave <beaub@...ux.microsoft.com>, Jens Remus <jremus@...ux.ibm.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v10 07/14] unwind_user/deferred: Make unwind deferral requests NMI-safe

On June 19, 2025 4:48:13 AM EDT, Peter Zijlstra <peterz@...radead.org> wrote:
>On Thu, Jun 19, 2025 at 10:44:27AM +0200, Peter Zijlstra wrote:
>
>> Luckily, x86 dropped support for !CMPXCHG8B right along with !TSC. So on
>> x86 we good with timestamps, even on 32bit.
>
>Well, not entirely true, local_clock() is not guaranteed monotonic. So
>you might be in for quite a bit of hurt if you rely on that.
>

As long as it is monotonic per task. If it is not, then pretty much all tracers that use it are broken.

-- Steve