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Message-ID: <20250123084026.GF3808@noisy.programming.kicks-ass.net>
Date: Thu, 23 Jan 2025 09:40:26 +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 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?
> > cookie = READ_ONCE(current->unwind_info.cookie);
> > do {
> > if (cookie)
> > return cookie;
> > cookie = ctx_to_cookie(cpu, ctr+1);
> > } while (!try_cmpxchg64(¤t->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.
We're going to be doing userspace stack unwinding, I don't think
overhead is a real concern.
> 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
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.
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