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

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Message-ID: <20250619094505.GC1613376@noisy.programming.kicks-ass.net>
Date: Thu, 19 Jun 2025 11:45:05 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Steven Rostedt <rostedt@...dmis.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 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.