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SubjectRe: [PATCH][RT] futex: protect against pi_blocked_on corruption during requeue PI
On Tue, 13 Jul 2010, Thomas Gleixner wrote:
> On Tue, 13 Jul 2010, Darren Hart wrote:
>
> > --- a/kernel/rtmutex.c
> > +++ b/kernel/rtmutex.c
> > @@ -227,7 +227,7 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
> > * reached or the state of the chain has changed while we
> > * dropped the locks.
> > */
> > - if (!waiter || !waiter->task)
> > + if (!waiter || (long)waiter == PI_WAKEUP_INPROGRESS || !waiter->task)
> > goto out_unlock_pi;
>
> Why do we need that check ? Either the requeue succeeded then
> task->pi_blocked_on is set to the real waiter or the wakeup won and
> we are in no lock chain.
>
> If we ever find a waiter with PI_WAKEUP_INPROGRESS set in
> rt_mutex_adjust_prio_chain() then it's a bug nothing else.

Grrr, I'm wrong. If we take hb->lock in the fast path then something
else might try to boost us and trip over this :(

This code causes braindamage. I really wonder whether we need to
remove it according to the "United Nations Convention against Torture
and Other Cruel, Inhuman or Degrading Treatment or Punishment".

> > @@ -6377,7 +6379,8 @@ void task_setprio(struct task_struct *p, int prio)
> > */
> > if (unlikely(p == rq->idle)) {
> > WARN_ON(p != rq->curr);
> > - WARN_ON(p->pi_blocked_on);
> > + WARN_ON(p->pi_blocked_on &&
> > + (long)p->pi_blocked_on != PI_WAKEUP_INPROGRESS);
>
> Yuck. Paranoia ? If we ever requeue idle, then .....

At least one which is bogus :)

Thanks,

tglx


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