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

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Date:	Sat, 17 Sep 2011 14:57:27 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Darren Hart <dvhart@...ux.intel.com>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Mike Galbraith <efault@....de>
Subject: Re: [RFC][PATCH 2/3] futex: Reduce hash bucket lock contention

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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