linux-kernel - Re: [RFC][PATCH 3/3] ipc/sem: Rework wakeup scheme

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Message-ID: <4E749467.4010309@colorfullife.com>
Date:	Sat, 17 Sep 2011 14:36:55 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 3/3] ipc/sem: Rework wakeup scheme

On 09/15/2011 09:45 PM, Peter Zijlstra wrote:
> On Thu, 2011-09-15 at 21:35 +0200, Peter Zijlstra wrote:
>> On Thu, 2011-09-15 at 21:32 +0200, Peter Zijlstra wrote:
>>>>> +static void wake_up_sem_queue_prepare(struct wake_list_head *wake_list,
>>>>>                              struct sem_queue *q, int error)
>>>>>    {
>>>>> +   struct task_struct *p = ACCESS_ONCE(q->sleeper);
>>>>>
>>>>> +   get_task_struct(p);
>>>>> +   q->status = error;
>>>>> +   /*
>>>>> +    * implies a full barrier
>>>>> +    */
>>>>> +   wake_list_add(wake_list, p);
>>>>> +   put_task_struct(p);
>>>>>    }
>>>> I think the get_task_struct()/put_task_struct is not necessary:
>>>> Just do the wake_list_add() before writing q->status:
>>>> wake_list_add() is identical to list_add_tail(&q->simple_list, pt).
>>>> [except that it contains additional locking, which doesn't matter here]
>> OK, I can't read properly.. so the problem with doing the
>> wake_list_add() before the write is that the wakeup can actually happen
>> before the write in case p already had a wakeup queued.
> Ah, but if the wakeup happens early, we return from schedule with -EINTR
> and re-acquire the sem_lock and re-test. Since we do this update from
> under sem_lock it should serialize and come out all-right,.. right?
Correct. Just think about a timeout of semtimedop().
The code handles early wakeup properly, it will either return -EAGAIN or 0.
(except for -EINTR).

--
     Manfred
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