linux-kernel - Re: [PATCH v10 07/14] unwind_user/deferred: Make unwind deferral requests NMI-safe

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Message-ID: <20250619084427.GA1613376@noisy.programming.kicks-ass.net>
Date: Thu, 19 Jun 2025 10:44:27 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	bpf@...r.kernel.org, x86@...nel.org,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrii Nakryiko <andrii@...nel.org>,
	Indu Bhagat <indu.bhagat@...cle.com>,
	"Jose E. Marchesi" <jemarch@....org>,
	Beau Belgrave <beaub@...ux.microsoft.com>,
	Jens Remus <jremus@...ux.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v10 07/14] unwind_user/deferred: Make unwind deferral
 requests NMI-safe

On Thu, Jun 19, 2025 at 04:37:33AM -0400, Steven Rostedt wrote:
> On Thu, 19 Jun 2025 10:34:15 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > Why can't we cmpxchg_local() the thing and avoid this horrible stuff?
> > 
> > static u64 get_timestamp(struct unwind_task_info *info)
> > {
> > 	u64 new, old = info->timestamp;
> > 
> > 	if (old)
> > 		return old;
> > 	
> > 	new = local_clock();
> > 	old = cmpxchg_local(&info->timestamp, old, new);
> > 	if (old)
> > 		return old;
> > 	return new;
> > }
> > 
> > Seems simple enough; what's wrong with it?
> 
> It's a 64 bit number where most 32 bit architectures don't have any
> decent cmpxchg on 64 bit values. That's given me hell in the ring
> buffer code :-p

Do we really have to support 32bit?

But IIRC a previous version of all this had a syscall counter. If you
make this a per task syscall counter, unsigned long is plenty.

I suppose that was dropped because adding that counter increment to all
syscalls blows. But if you really want to support 32bit, that might be a
fallback.

Luckily, x86 dropped support for !CMPXCHG8B right along with !TSC. So on
x86 we good with timestamps, even on 32bit.