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

Manfred Spraul
2011-09-17T12:30:42Z

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