Re: [RFC][PATCH 2/3] futex: Reduce hash bucket lock contention

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Wed, 2011-09-14 at 15:30 +0200, Peter Zijlstra wrote:
> @@ -964,6 +961,7 @@ futex_wake(u32 __user *uaddr, unsigned i
>         struct futex_q *this, *next;
>         struct plist_head *head;
>         union futex_key key = FUTEX_KEY_INIT;
> +       WAKE_LIST(wake_list);
>         int ret;
>  
>         if (!bitset)
> @@ -988,7 +986,7 @@ futex_wake(u32 __user *uaddr, unsigned i
>                         if (!(this->bitset & bitset))
>                                 continue;
>  
> -                       wake_futex(this);
> +                       wake_futex(&wake_list, this);
>                         if (++ret >= nr_wake)
>                                 break;
>                 }
> @@ -996,6 +994,8 @@ futex_wake(u32 __user *uaddr, unsigned i
>  
>         spin_unlock(&hb->lock);
>         put_futex_key(&key);
> +
> +       wake_up_list(&wake_list, TASK_NORMAL);
>  out:
>         return ret;
>  } 

So while initially I thought the sem patch was busted, it turns out this
one is.

Thomas managed to spot the race:

  Task-0			Task-1

futex_wait()
  queue_me()

				futex_wake()
				  wake_list_add();
				  __unqueue_futex();
				    plist_del();
  if (!plist_node_empty())
  __set_current_state(TASK_RUNNNIG);

				  wake_up_list();
				    /* waking an already running task-0 */


I guess the biggest question is, do we care? Ideally everything should
be able to deal with spurious wakeups, although we generally try to
avoid them.