linux-kernel - Re: [RFC PATCH v3 7/9] yield_to(), cpu-hotplug: Prevent offlining of other CPUs properly

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Date:	Sun, 9 Dec 2012 20:48:26 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	tglx@...utronix.de, peterz@...radead.org,
	paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
	mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org,
	vincent.guittot@...aro.org, tj@...nel.org, sbw@....edu,
	amit.kucheria@...aro.org, rostedt@...dmis.org, rjw@...k.pl,
	wangyun@...ux.vnet.ibm.com, xiaoguangrong@...ux.vnet.ibm.com,
	nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v3 7/9] yield_to(), cpu-hotplug: Prevent offlining
	of other CPUs properly

On 12/07, Srivatsa S. Bhat wrote:
>
> Once stop_machine() is gone from the CPU offline path, we won't be able to
> depend on local_irq_save() to prevent CPUs from going offline from under us.

OK, I guess we need to avoid resched_task()->smp_send_reschedule()
after __cpu_disable() and before migrate_tasks().

But, whatever problem we have,

> Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going offline,
> while invoking from atomic context.

it should be solved, so...

> -		if (preempt && rq != p_rq)
> +		if (preempt && rq != p_rq && cpu_online(task_cpu(p)))

Why do we need this change?

Afaics, you could add BUG_ON(!cpu_online(...)) instead?

I am just curious.

Oleg.

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