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

[Manfred Spraul]

On 09/16/2011 02:34 PM, Peter Zijlstra wrote:
> 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.
The sem patch also causes such wakeups:

 Task-0                Task-1
 semtimedop()
  schedule_timeout()

                            semtimedop()
                            wake_list_add();
                            q->status = 0;
<Timeout>
  schedule_timeout() returns
  if (q->status==0)
    return;
 semtimedop() returns

 random user space/kernel space code

                            spin_unlock();
                            wake_up_list();

It's a rare event, but that does happen.
Which means:
How do we verify that everything is able to deal with spurious wakeups?

--
    Manfred
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