linux-kernel - Re: getting processor numbers

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Date:	Wed, 4 Apr 2007 12:57:00 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ulrich Drepper <drepper@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Gautham R Shenoy <ego@...ibm.com>,
	Dipankar Sarma <dipankar@...ibm.com>, Paul Jackson <pj@....com>
Subject: Re: getting processor numbers

On 04/04, Ingo Molnar wrote:
> 
> * Oleg Nesterov <oleg@...sign.ru> wrote:
> 
> > But we don't need tasklist_lock at all, we can use 
> > rcu_read_lock/unlock. Q: don't we need task_rq_lock() to read 
> > ->cpus_allowed "atomically" ?
> 
> right now ->cpus_allowed is protected by tasklist_lock. We cannot do RCU 
> here because ->cpus_allowed modifications are not RCUified.

Is it so? that was my question. Afaics, set_cpus_allowed() does
p->cpus_allowed = new_mask under rq->lock, so I don't understand
how tasklist_lock can help.

Could you clarify?

Oleg.

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