linux-kernel - Re: [PATCH 2/4] vfork: make it killable

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Date:	Fri, 17 Feb 2012 15:44:58 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	apw@...onical.com, arjan@...ux.intel.com, fhrbata@...hat.com,
	john.johansen@...onical.com, penguin-kernel@...ove.SAKURA.ne.jp,
	rientjes@...gle.com, rusty@...tcorp.com.au, tj@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] vfork: make it killable

On 02/16, Andrew Morton wrote:
>
> On Thu, 16 Feb 2012 18:27:06 +0100
> Oleg Nesterov <oleg@...hat.com> wrote:
>
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -669,10 +669,34 @@ struct mm_struct *mm_access(struct task_struct *task, unsigned int mode)
> >
> >  void complete_vfork_done(struct task_struct *tsk)
> >  {
> > -	struct completion *vfork_done = tsk->vfork_done;
> > +	struct completion *vfork;
> >
> > -	tsk->vfork_done = NULL;
> > -	complete(vfork_done);
> > +	task_lock(tsk);
> > +	vfork = tsk->vfork_done;
> > +	if (likely(vfork)) {
> > +		tsk->vfork_done = NULL;
> > +		complete(vfork);
> > +	}
> > +	task_unlock(tsk);
> > +}
>
> OK, so now we don't need to test tsk->vfork_done in callers.  But
> mm_release() still does this, and it does it outside locks.

Yes, complete_vfork_done() can be called unconditionally,

> Mistake,
> or micro-optimisation?

micro-optimisation to avoid the unnecessary task_lock().

> If the latter, why is the lockless peek
> race-free?

If ->vfork_done != NULL, the child can never miss it. The parent
sets this pointer before the first wakeup.

However. The killed parent can clear ->vfork_done, see "if (killed)"
int wait_for_vfork_done(). That is why complete_vfork_done() should
re-check under task_lock().

Oleg.

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