linux-kernel - Re: [PATCH v4 30/39] unwind_user/deferred: Make unwind deferral requests NMI-safe

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Message-ID: <20250123221734.GE969@noisy.programming.kicks-ass.net>
Date: Thu, 23 Jan 2025 23:17:34 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: x86@...nel.org, Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	linux-kernel@...r.kernel.org, Indu Bhagat <indu.bhagat@...cle.com>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	linux-perf-users@...r.kernel.org, Mark Brown <broonie@...nel.org>,
	linux-toolchains@...r.kernel.org, Jordan Rome <jordalgo@...a.com>,
	Sam James <sam@...too.org>, linux-trace-kernel@...r.kernel.org,
	Andrii Nakryiko <andrii.nakryiko@...il.com>,
	Jens Remus <jremus@...ux.ibm.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Florian Weimer <fweimer@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Weinan Liu <wnliu@...gle.com>
Subject: Re: [PATCH v4 30/39] unwind_user/deferred: Make unwind deferral
 requests NMI-safe

On Thu, Jan 23, 2025 at 11:48:07AM -0800, Josh Poimboeuf wrote:
> > > > 	cookie = READ_ONCE(current->unwind_info.cookie);
> > > > 	do {
> > > > 		if (cookie)
> > > > 			return cookie;
> > > > 		cookie = ctx_to_cookie(cpu, ctr+1);
> > > > 	} while (!try_cmpxchg64(&current->unwind_info.cookie, &cookie, cookie));
> 
> Should not the 2nd argument be &zero?

This I suppose

	cookie = READ_ONCE(current->unwind_info.cookie);
	do {
		if (cookie)
			return cookie;
	} while (!try_cmpxchg64(&current->unwind_info.cookie, &cookie,
				ctx_to_cookie(cpu, ctr+1)));