linux-kernel - Re: [RFC PATCH 4/5] Remove CPU_DEAD/CPU_UP

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Date:	Wed, 24 Oct 2007 17:44:04 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	ego@...ibm.com, Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC PATCH 4/5] Remove CPU_DEAD/CPU_UP_CANCELLED handling from workqueue.c

On 10/24, Rusty Russell wrote:
>
> On Wednesday 24 October 2007 15:37:16 Gautham R Shenoy wrote:
> > @@ -712,7 +712,7 @@ static void start_workqueue_thread(struc
> >
> >  	if (p != NULL) {
> >  		if (cpu >= 0)
> > -			kthread_bind(p, cpu);
> > +			set_cpus_allowed(p, cpumask_of_cpu(cpu));
> >  		wake_up_process(p);
> >  	}
> 
> Hi Gautham!
> 
> 	This works, although the change is unnecessary.

kthread_bind() changes ->cpu/->cpus_allowed without any locks.

Currently this is possible because nobody can wakeup the new thread
after wait_task_inactive() succeeds. With this patch this is not true
any longer, cwq->thread can sleep, for example, on cwq->more_work.

Oleg.

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