linux-kernel - Re: lguest, Re: -mm merge plans for 2.6.23

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Date:	Wed, 11 Jul 2007 21:24:35 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	David Miller <davem@...emloft.net>, hch@....de,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: lguest, Re: -mm merge plans for 2.6.23

On Thu, 12 Jul 2007 12:48:41 +1000 Rusty Russell <rusty@...tcorp.com.au> wrote:

> On Wed, 2007-07-11 at 19:28 -0700, David Miller wrote:
> > From: Rusty Russell <rusty@...tcorp.com.au>
> > Date: Thu, 12 Jul 2007 11:21:51 +1000
> > 
> > > To do inter-guest (ie. inter-process) I/O you really have to make sure
> > > the other side doesn't go away.
> > 
> > You should just let it exit and when it does you receive some kind of
> > exit notification that resets your virtual device channel.
> > 
> > I think the reference counting approach is error and deadlock prone.
> > Be more loose and let the events reset the virtual devices when
> > guests go splat.
> 
> There are two places where we grab task refcnt.  One might be avoidable
> (will test and get back) but the deferred wakeup isn't really:
> 
>         /* We cache one process to wakeup: helps for batching & wakes outside locks. */
>         void set_wakeup_process(struct lguest *lg, struct task_struct *p)
>         {
>         	if (p == lg->wake)
>         		return;
>         
>         	if (lg->wake) {
>         		wake_up_process(lg->wake);
>         		put_task_struct(lg->wake);
>         	}
>         	lg->wake = p;
>         	if (lg->wake)
>         		get_task_struct(lg->wake);
>         }

<handwaving>

We seem to be taking the reference against the wrong thing here.  It should
be against the mm, not against a task_struct?
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