On Tue, Mar 07, 2017 at 03:31:50PM +0100, Thomas Gleixner wrote:
> On Sat, 4 Mar 2017, Peter Zijlstra wrote:
>
> > The problem with returning -EAGAIN when the waiter state mismatches is
> > that it becomes very hard to proof a bounded execution time on the
> > operation. And seeing that this is a RT operation, this is somewhat
> > important.
> >
> > While in practise it will be very unlikely to ever really take more
> > than one or two rounds, proving so becomes rather hard.
>
> Oh no. Assume the following:
>
> T1 and T2 are both pinned to CPU0. prio(T2) > prio(T1)
>
> CPU0
>
> T1
> lock_pi()
> queue_me() <- Waiter is visible
>
> preemption
>
> T2
> unlock_pi()
> loops with -EAGAIN forever
Ah! indeed.
> > Now that modifying wait_list is done while holding both hb->lock and
> > wait_lock, we can avoid the scenario entirely if we acquire wait_lock
> > while still holding hb-lock. Doing a hand-over, without leaving a
> > hole.
>
> > Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
> > ---
> > kernel/futex.c | 26 ++++++++++++--------------
> > 1 file changed, 12 insertions(+), 14 deletions(-)
> >
> > --- a/kernel/futex.c
> > +++ b/kernel/futex.c
> > @@ -1391,16 +1391,11 @@ static int wake_futex_pi(u32 __user *uad
> > DEFINE_WAKE_Q(wake_q);
> > int ret = 0;
> >
> > new_owner = rt_mutex_next_owner(&pi_state->pi_mutex);
> > + if (WARN_ON_ONCE(!new_owner)) {
> > /*
> > + * Should be impossible now... but if weirdness happens,
>
> 'now...' is not very useful 6 month from NOW :)
I'll put in a reference to the below comment in, that explains why this
should now be impossible.
> > + * returning -EAGAIN is safe and correct.
> > */
> > ret = -EAGAIN;
> > goto out_unlock;
> > @@ -2770,15 +2765,18 @@ static int futex_unlock_pi(u32 __user *u
> > if (pi_state->owner != current)
> > goto out_unlock;
> >
> > + get_pi_state(pi_state);
> > /*
> > + * Since modifying the wait_list is done while holding both
> > + * hb->lock and wait_lock, holding either is sufficient to
> > + * observe it.
> > *
> > + * By taking wait_lock while still holding hb->lock, we ensure
> > + * there is no point where we hold neither; and therefore
> > + * wake_futex_pi() must observe a state consistent with what we
> > + * observed.
> > */
^^ that one.
> > + raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
> > spin_unlock(&hb->lock);
