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SubjectRe: lguest, Re: -mm merge plans for 2.6.23
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On Wed, 2007-07-11 at 19:28 -0700, David Miller wrote:
> From: Rusty Russell <rusty@rustcorp.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);
}
We drop the lock after I/O, and then do this wakeup. Meanwhile the
other task might have exited.

I could get rid of it, but I don't think there's anything wrong with the
code...

Cheers,
Rusty.


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