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Date: Wed, 4 Apr 2007 12:57:00 +0400
From: Oleg Nesterov <oleg@tv-sign.ru>
To: Ingo Molnar <mingo@elte.hu>
Cc: Andrew Morton <akpm@linux-foundation.org>,
       Ulrich Drepper <drepper@redhat.com>,
       Linux Kernel <linux-kernel@vger.kernel.org>,
       Gautham R Shenoy <ego@in.ibm.com>, Dipankar Sarma <dipankar@in.ibm.com>,
       Paul Jackson <pj@sgi.com>
Subject: Re: getting processor numbers
Message-ID: <20070404085700.GA170@tv-sign.ru>
References: <461286D6.2040407@redhat.com> <20070403131623.c6831607.akpm@linux-foundation.org> <4612BB89.8040102@redhat.com> <20070403141348.9bcdb13e.akpm@linux-foundation.org> <4612D175.30604@redhat.com> <20070403154831.37bde672.akpm@linux-foundation.org> <4612DCA2.2090400@redhat.com> <20070403162349.583adf84.akpm@linux-foundation.org> <20070404083927.GA100@tv-sign.ru> <20070404093948.GA14521@elte.hu>
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On 04/04, Ingo Molnar wrote:
> 
> * Oleg Nesterov <oleg@tv-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.