linux-kernel - Re: [PATCH v2] smp: Document preemption and stop

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Message-ID: <20250707075050.GB1613200@noisy.programming.kicks-ass.net>
Date: Mon, 7 Jul 2025 09:50:50 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Joel Fernandes <joelagnelf@...dia.com>
Cc: linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Andrea Righi <arighi@...dia.com>,
	"Paul E . McKenney" <paulmck@...nel.org>,
	Frederic Weisbecker <frederic@...nel.org>, rcu@...r.kernel.org
Subject: Re: [PATCH v2] smp: Document preemption and stop_machine() mutual
 exclusion

On Sat, Jul 05, 2025 at 01:23:27PM -0400, Joel Fernandes wrote:
> Recently while revising RCU's cpu online checks, there was some discussion
> around how IPIs synchronize with hotplug.
> 
> Add comments explaining how preemption disable creates mutual exclusion with
> CPU hotplug's stop_machine mechanism. The key insight is that stop_machine()
> atomically updates CPU masks and flushes IPIs with interrupts disabled, and
> cannot proceed while any CPU (including the IPI sender) has preemption
> disabled.

I'm very conflicted on this. While the added comments aren't wrong,
they're not quite accurate either. Stop_machine doesn't wait for people
to enable preemption as such.

Fundamentally there seems to be a misconception around what stop machine
is and how it works, and I don't feel these comments make things better.

Basically, stop-machine (and stop_one_cpu(), stop_two_cpus()) use the
stopper task, a task running at the ultimate priority; if it is
runnable, it will run.

Stop-machine simply wakes all the stopper tasks and co-ordinates them to
literally stop the machine. All CPUs have the stopper task scheduled and
then they go sit in a spin-loop driven state machine with IRQs disabled.

There really isn't anything magical about any of this.