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Subject: Re: [RFC][PATCH 07/17] sched: Serialize p->cpus_allowed and ttwu()
 using p->pi_lock
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Yong Zhang <yong.zhang0@gmail.com>
Cc: Chris Mason <chris.mason@oracle.com>,
        Frank Rowand <frank.rowand@am.sony.com>, Ingo Molnar <mingo@elte.hu>,
        Thomas Gleixner <tglx@linutronix.de>, Mike Galbraith <efault@gmx.de>,
        Oleg Nesterov <oleg@redhat.com>, Paul Turner <pjt@google.com>,
        Jens Axboe <axboe@kernel.dk>, linux-kernel@vger.kernel.org
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References: <20101224122338.172750730@chello.nl>
	 <20101224123742.833040935@chello.nl>  <20101229142019.GB2728@zhy>
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Date: Mon, 03 Jan 2011 12:12:14 +0100
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On Wed, 2010-12-29 at 22:20 +0800, Yong Zhang wrote:
> > - * The caller (fork, wakeup) owns TASK_WAKING, ->cpus_allowed is stable.
> > + * The caller (fork, wakeup) owns p->pi_lock, ->cpus_allowed is stable.
> 
> Yes for wakeup, but not true for fork.
> I don't see protection in wake_up_new_task().
> Or am I missing something? 

Ah, true, wake_up_new_task() holds task_rq_lock() which is sufficient,
but yes, we could also make that pi_lock.