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

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Message-ID: <20250918151018.7281647b@batman.local.home>
Date: Thu, 18 Sep 2025 15:10:18 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: 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>, Josh Poimboeuf <jpoimboe@...nel.org>,
 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, 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.

-- Steve