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

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Message-ID: <20250123195451.f2a4jfu5tkfklwf2@jpoimboe>
Date: Thu, 23 Jan 2025 11:54:51 -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 11:48:10AM -0800, Josh Poimboeuf wrote:
> 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:
> > > 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)

Though, assuming we're keeping the unwind_work struct, there's a simpler
alternative to nmi_cookie: store the cookie in the unwind_work.  Then
the task work can just use that instead of current->unwind_info.cookie.

-- 
Josh