Re: [PATCH, RFC] kthread: (possibly) a missing memory barrier in kthread_stop()

[Dmitry Adamushko]

On 19/02/2008, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> wrote:
>  [ ... ]
> > >
> > > From: Dmitry Adamushko <dmitry.adamushko@xxxxxxxxx>
> > > Subject: kthread: add a memory barrier to kthread_stop()
> > >
> > > 'kthread' threads do a check in the following order:
> > > - set_current_state(TASK_INTERRUPTIBLE);
> > > - kthread_should_stop();
> > >
> > > and set_current_state() implies an smp_mb().
> > >
> > > on another side (kthread_stop), wake_up_process() is not guaranteed to
> > > act as a full mb.
> > >
> > > 'kthread_stop_info.k' must be visible before wake_up_process() checks
> > > for/modifies a state of the 'kthread' task.
> > >
> > >
> > > Signed-off-by: Dmitry Adamushko <dmitry.adamushko@xxxxxxxxx>
> > >
> > >
> > > diff --git a/kernel/kthread.c b/kernel/kthread.c
> > > index 0ac8878..5167110 100644
> > > --- a/kernel/kthread.c
> > > +++ b/kernel/kthread.c
> > > @@ -211,6 +211,10 @@ int kthread_stop(struct task_struct *k)
> > >
> > >     /* Now set kthread_should_stop() to true, and wake it up. */
> > >     kthread_stop_info.k = k;
> > > +
> > > +   /* The previous store operation must not get ahead of the wakeup. */
> > > +   smp_mb();
>
> Does this not also imply you need a matching barrier in
> kthread_should_stop() ?

Yes, but only when it's used in combination with something that alters
a state of the task.

So it's rather a question of the interface-design.

We currently impose a requirement on how a main loop of 'kthread'
threads (ok, so it seems to dictate a policy :-) has to be orginized.
Namely, the following sequence must be kept in order:

(1) set_current_task(TASK_INTERRUPTIBLE);
(2) kthread_should_stop()
...
- schedule()

and (1) already provides a mb which becomes a "matching barrier" on
the kthread_should_stop() side.


-- 
Best regards,
Dmitry Adamushko
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