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Date: Wed, 24 Mar 2010 19:09:12 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@elte.hu>, Ben Blum <bblum@google.com>,
       Jiri Slaby <jirislaby@gmail.com>, Lai Jiangshan <laijs@cn.fujitsu.com>,
       Li Zefan <lizf@cn.fujitsu.com>, Miao Xie <miaox@cn.fujitsu.com>,
       Paul Menage <menage@google.com>, "Rafael J. Wysocki" <rjw@sisk.pl>,
       Tejun Heo <tj@kernel.org>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/6] sched/cpusets fixes, more changes are needed
Message-ID: <20100324180912.GA21774@redhat.com>
References: <20100315090958.GA9116@redhat.com> <1269452296.5109.508.camel@twins>
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On 03/24, Peter Zijlstra wrote:
>
> On Mon, 2010-03-15 at 10:09 +0100, Oleg Nesterov wrote:
> >
> > 	- do_fork() clears PF_STARTING and then calls wake_up_new_task()
> > 	  which finally does s/WAKING/RUNNING.
> >
> > 	  But. Nobody can take rq->lock in between. This means a signal
> > 	  from irq (quite possible with CLONE_THREAD) or another rt
> > 	  thread which preempts us can lockup.
>
> Hmm, the signal case might indeed be a problem, however I cannot see how
> the RT thread can be a problem because until we do wake_up_new_task()
> the child will not be runnable and can thus not be preempted.

Indeed, but I meant the _parent_ can be preempted ;)

In short. TASK_WAKING acts as a spinlock in fact. And since ttwu() can
be called from any context, it should be irq-safe: any owner must disable
inerrupts and preemption.

> The reason we have that TASK_WAKING stuff for fork is because
> wake_up_new_task() needs p->cpus_allowed to be stable

Sure! But it is very easy to change wake_up_new_task() to set TASK_WAKING
like ttwu() does. Of course, this needs raw_spin_lock_irq(rq->lock) for
a moment, but afaics that is all?

> So the below patch makes select_task_rq_fair unlock the rq when needed,
> and then puts all ->select_task_rq() calls under rq->lock. This should
> allow us to remove the TASK_WAKING thing from fork which in turn allows
> us to remove the PF_STARTING check in task_is_waking.
>
> How does that look?

I'll try to read this patch tomorrow. But could you please consider
the suggestion above?

Oleg.