linux-kernel - Re: [RESEND][PATCH v15 0/4] perf: Support the deferred unwinding infrastructure

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Message-ID: <pde32olzdlqvbom5bya5exndcrfgsw7lmffy6uav5yoplonzj3@ddb2b5sihlpx>
Date: Fri, 19 Sep 2025 16:34:02 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Peter Zijlstra <peterz@...radead.org>, 
	Steven Rostedt <rostedt@...nel.org>, 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>, Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...nel.org>, 
	Arnaldo Carvalho de Melo <acme@...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>, 
	Florian Weimer <fweimer@...hat.com>, Sam James <sam@...too.org>, Kees Cook <kees@...nel.org>, 
	Carlos O'Donell <codonell@...hat.com>
Subject: Re: [RESEND][PATCH v15 0/4] perf: Support the deferred unwinding
 infrastructure

On Thu, Sep 18, 2025 at 03:10:18PM -0400, Steven Rostedt wrote:
> On Thu, 18 Sep 2025 19:32:20 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > > Now, task_work_run() is in the exit_to_user_mode_loop() which is notably
> > > *before* exit_to_user_mode() which does the unwind_reset_info().
> > > 
> > > What happens if we get an NMI requesting an unwind after
> > > unwind_reset_info() while still very much being in the kernel on the way
> > > out?  
> > 
> > AFAICT it will try and do a task_work_add(TWA_RESUME) from NMI context,
> > and this will fail horribly.
> > 
> > If you do something like:
> > 
> > 	twa_mode = in_nmi() ? TWA_NMI_CURRENT : TWA_RESUME;
> > 	task_work_add(foo, twa_mode);
> > 
> > it might actually work.
> 
> Ah, the comment for TWA_RESUME didn't express this restriction.
> 
> That does look like that would work as the way I expected task_work to
> handle this case.

BTW, I remember Peter had a fix for TWA_NMI_CURRENT, I guess it got lost
in the shuffle or did something else happen in the meantime?

  https://lore.kernel.org/20250122124228.GO7145@noisy.programming.kicks-ass.net

-- 
Josh