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

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Message-ID: <20250123194807.2mn54dsk2ef5i4iy@jpoimboe>
Date: Thu, 23 Jan 2025 11:48:07 -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 Thu, Jan 23, 2025 at 09:40:26AM +0100, Peter Zijlstra wrote:
> On Wed, Jan 22, 2025 at 02:49:02PM -0800, Josh Poimboeuf wrote:
> > 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.
> 
> What for?

Hm, I guess it's not needed for this 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));

Should not the 2nd argument be &zero?

> > > 	__this_cpu_write(unwind_ctx_ctr, ctr+1);
> > > 	return cookie;
> > 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.
> 
> So how about we clear cookie (and set nr_entries to -1) at

I think we could set nr_entries to 0 instead of -1?

> return-to-user, after we've done the work loop and have interrupts
> disabled until we hit userspace.
>
> Any NMI that hits there will have to cause another entry anyway.

But there's a cookie mismatch:

    // return-to-user: IRQs disabled
    <NMI>
	current->unwind_info.cookie = 0x1234
    </NMI>
    unwind_exit_to_user_mode()
	current->unwind_info.cookie = 0
    IRET
<IRQ>
    task_work()
        callback(@cookie=WRONG)

-- 
Josh