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

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Message-ID: <20250619043733.2a74d431@batman.local.home>
Date: Thu, 19 Jun 2025 04:37:33 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Peter Zijlstra <peterz@...radead.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, 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

-- Steve