Re: [PATCH -v5 14/14] futex: futex_unlock_pi() determinism

[Peter Zijlstra]

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

So this is true before the last patch; but if we look at the locking
changes brought by that (pasting its changelog here):

Before:

futex_lock_pi()                 futex_unlock_pi()
  unlock hb->lock

                                  lock hb->lock
                                  unlock hb->lock

                                  lock rt_mutex->wait_lock
                                  unlock rt_mutex_wait_lock
                                    -EAGAIN

  lock rt_mutex->wait_lock
  list_add
  unlock rt_mutex->wait_lock

  schedule()

  lock rt_mutex->wait_lock
  list_del
  unlock rt_mutex->wait_lock

                                  <idem>
                                    -EAGAIN

  lock hb->lock


After:

futex_lock_pi()                 futex_unlock_pi()

  lock hb->lock
  lock rt_mutex->wait_lock
  list_add
  unlock rt_mutex->wait_lock
  unlock hb->lock

  schedule()
                                  lock hb->lock
                                  unlock hb->lock
  lock hb->lock
  lock rt_mutex->wait_lock
  list_del
  unlock rt_mutex->wait_lock

                                  lock rt_mutex->wait_lock
                                  unlock rt_mutex_wait_lock
                                    -EAGAIN

  unlock hb->lock


Your T2 (of higher prio) would block on T1's hb->lock and boost T1
(since hb->lock is an rt_mutex).

Alternatively (!PREEMPT_FULL), the interleave cannot happen (when pinned
to a single CPU) because then hb->lock disables preemption, it being a
spinlock.


Unless I need to go drink more wake-up-juice..