Re: [PATCH] fix refrigerator() vs thaw_process() race

From: Oleg Nesterov
Date: Mon Feb 19 2007 - 07:13:47 EST


On 02/19, Pavel Machek wrote:
>
> > refrigerator() can miss a wakeup, "wait event" loop needs a proper memory
> > ordering.
> >
> > Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> >
> > --- WQ/kernel/power/process.c~WAKE 2007-02-18 22:56:49.000000000 +0300
> > +++ WQ/kernel/power/process.c 2007-02-19 01:04:26.000000000 +0300
> > @@ -46,8 +46,10 @@ void refrigerator(void)
> > recalc_sigpending(); /* We sent fake signal, clean it up */
> > spin_unlock_irq(&current->sighand->siglock);
> >
> > - current->state = TASK_UNINTERRUPTIBLE;
> > + set_current_state(TASK_UNINTERRUPTIBLE);
>
>
> Looks okay to me... but this one liner would be exactly as effective,
> right?

I think no, with this one liner we have

while (frozen(current)) {
// ------ WINDOW ------------
set_current_state(TASK_UNINTERRUPTIBLE);
schedule();
}

What if thaw_process() happens in the window above?

We need the barrier exactly because LOAD (check condition) should not
come before STORE (set task->state).

Oleg.

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