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

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Message-ID: <20250122224902.hhpmhi7r3azz3a4x@jpoimboe>
Date: Wed, 22 Jan 2025 14:49:02 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Peter Zijlstra <peterz@...radead.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 Wed, Jan 22, 2025 at 03:15:05PM +0100, Peter Zijlstra wrote:
> On Tue, Jan 21, 2025 at 06:31:22PM -0800, Josh Poimboeuf wrote:
> Oh gawd. Can we please do something simple like:
> 
> 	guard(irqsave)();
> 	cpu = raw_smp_processor_id();
> 	ctr = __this_cpu_read(unwind_ctx_cnt);

Don't you need a compiler barrier here?  __this_cpu_read() doesn't have
one.

> 	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));
> 	__this_cpu_write(unwind_ctx_ctr, ctr+1);
> 	return cookie;

I was trying to avoid the overhead of the cmpxchg.

But also, the nmi_cookie is still needed for the case where the NMI
arrives before info->cookie gets cleared by early entry-from-user.

-- 
Josh