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

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Message-ID: <66A7F6C1-3693-4F76-A513-7CBBE3154B06@goodmis.org>
Date: Thu, 19 Jun 2025 06:19:28 -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 5:45:05 AM EDT, Peter Zijlstra <peterz@...radead.org> wrote:
>On Thu, Jun 19, 2025 at 05:42:31AM -0400, Steven Rostedt wrote:
>> 
>> 
>> On June 19, 2025 5:32:26 AM EDT, Peter Zijlstra <peterz@...radead.org> wrote:
>> >On Thu, Jun 19, 2025 at 05:07:10AM -0400, Steven Rostedt wrote:
>> >
>> >> Does #DB make in_nmi() true? If that's the case then we do need to handle that.
>> >
>> >Yes: #DF, #MC, #BP (int3), #DB and NMI all have in_nmi() true.
>> >
>> >Ignoring #DF because that's mostly game over, you can get them all
>> >nested for up to 4 (you're well aware of the normal NMI recursion
>> >crap).
>> 
>> We probably can implement this with stacked counters.
>
>I would seriously consider dropping support for anything that can't do
>cmpxchg at the width you need.

That may be something we can do as it's a new feature and unlike the ftrace ring buffer, it won't be a regression not to support them.

We currently care about x86-64, arm64, ppc 64 and s390. I'm assuming they all have a proper 64 bit cmpxchg.

-- Steve