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

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Message-ID: <20250122142418.GV7145@noisy.programming.kicks-ass.net>
Date: Wed, 22 Jan 2025 15:24:18 +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 Tue, Jan 21, 2025 at 06:31:22PM -0800, Josh Poimboeuf wrote:
> +static int unwind_deferred_request_nmi(struct unwind_work *work, u64 *cookie)
> +{
> +	struct unwind_task_info *info = &current->unwind_info;
> +	bool inited_cookie = false;
> +	int ret;
> +
> +	*cookie = info->cookie;
> +	if (!*cookie) {
> +		/*
> +		 * This is the first unwind request since the most recent entry
> +		 * from user.  Initialize the task cookie.
> +		 *
> +		 * Don't write to info->cookie directly, otherwise it may get
> +		 * cleared if the NMI occurred in the kernel during early entry
> +		 * or late exit before the task work gets to run.  Instead, use
> +		 * info->nmi_cookie which gets synced later by get_cookie().
> +		 */
> +		if (!info->nmi_cookie) {
> +			u64 cpu = raw_smp_processor_id();
> +			u64 ctx_ctr;
> +
> +			ctx_ctr = __this_cpu_inc_return(unwind_ctx_ctr);

__this_cpu_inc_return() is *NOT* NMI safe IIRC.