linux-kernel - Re: [PATCH] tracing: Remove pointless memory barriers

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Message-ID: <20250709110827.0dce4012@batman.local.home>
Date: Wed, 9 Jul 2025 11:08:27 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Nam Cao <namcao@...utronix.de>
Cc: Gabriele Monaco <gmonaco@...hat.com>, John Ogness
 <john.ogness@...utronix.de>, Masami Hiramatsu <mhiramat@...nel.org>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 linux-trace-kernel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tracing: Remove pointless memory barriers

On Tue, 8 Jul 2025 09:42:19 +0200
Nam Cao <namcao@...utronix.de> wrote:

> So yes, smp_rmb() is only useful inbetween reads, and smp_wmb() is
> only userful inbetween writes.

Hmm, I wonder if barriers isn't needed but atomic values are?

That is, it looks like rv_monitoring_on() is looking to read the
current state, where as turn_monitoring_on/off() changes the state.

Perhaps instead of barriers, it should use atomics?

 bool rv_monitoring_on(void)
 {
	return atomic_read(&monitoring_on);
 }

 static void turn_monitoring_off(void)
 {
	atomic_set(&monitoring_on, 0);
 }

Doesn't atomic make sure the values are seen when they are changed?

As this code is more about looking at state and not ordering, and I
think that's what atomics are about.

-- Steve