linux-kernel - Re: [RFC PATCH v1] stop_machine: Add stop_housekeeping

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Message-ID: <20241218172754.GP2354@noisy.programming.kicks-ass.net>
Date: Wed, 18 Dec 2024 18:27:54 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Costa Shulyupin <costa.shul@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Yury Norov <yury.norov@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Valentin Schneider <vschneid@...hat.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	linux-kernel@...r.kernel.org, Waiman Long <longman@...hat.com>,
	x86@...nel.org
Subject: Re: [RFC PATCH v1] stop_machine: Add stop_housekeeping_cpuslocked()

On Wed, Dec 18, 2024 at 07:15:31PM +0200, Costa Shulyupin wrote:

> Which synchronizations do these functions require instead of stop_machine?

*sigh*, so you're asking us to do your homework?

But clearly you're not realizing the scope of the thing: stop_machine()
serializes against every preempt_disable() region in the entire kernel.

So you're telling me there isn't a single preempt_disable() region in
the kernel that depends on being before stop_machine() in its entirety?

I know for a fact I've written some in the past 20 years -- what I don't
know if any of them still live and are still relying on it, because I've
also added synchronize_rcu_sched(), which later became synchronize_rcu()
which implies the same, in various parts of the hotplug machinery.

Anyway, without you doing some proper analysis, your patches are going
exactly nowhere.