linux-kernel - Re: [RFC PATCH 0/6] stop_machine: kill stop_cpus_mutex and stop_cpus

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Message-ID: <20150630010811.GB22559@redhat.com>
Date:	Tue, 30 Jun 2015 03:08:11 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	paulmck@...ux.vnet.ibm.com, tj@...nel.org, mingo@...hat.com,
	der.herr@...r.at, dave@...olabs.net, riel@...hat.com,
	viro@...IV.linux.org.uk, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/6] stop_machine: kill stop_cpus_mutex and
	stop_cpus_lock

On 06/29, Peter Zijlstra wrote:
>
> On Mon, Jun 29, 2015 at 06:02:51AM +0200, Oleg Nesterov wrote:
>
> > If we change stop_two_cpus() to use stop_work_alloc_one() it can use
> > cpu_online(),
>
> So the one user of this actually needs cpu_active(); we do not want to
> go move tasks to an inactive cpu.
>
> So if you change this to cpu_online() we need to audit the user is doing
> the stricter test.

Hmm. But the user (migrate_swap_stop) should check cpu_active() anyway?

The cpu_active() checks in stop_two_cpus() can only help to ensure that
multi_cpu_stop() won't hang. CPU_DOWN_PREPARE can deactivate either CPU
right after the check?

Or stop_two_cpus() needs get_online_cpus(). Or I missed something.

Oleg.

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