Re: [RFC v4] wait: prevent waiter starvation in __wait_on_bit_lock

[Oleg Nesterov <oleg@redhat.com>]

On 01/23, Dmitry Adamushko wrote:
>
> 2009/1/22 Oleg Nesterov <oleg@redhat.com>:
> >
> > I think this is correct, and (unfortunately ;) you are right:
> > we need rmb() even after finish_wait().
>
> Hum, I think it's actually not necessary in this particular case when
> (1) "the next contender is us" and (2) we are in the "ret != 0" path
> so that the only thing we really care about -- if we were exclusivly
> woken up, then wake up somebody else [*].
>
> "the next contender is us" implies that we were still on the 'wq'
> queue when __wake_up_bit() -> __wake_up() has been called, meaning
> that wq->lock has also been taken (in __wake_up()).
>
> Now, on our side, we are definitely on the 'wq' queue before calling
> finish_wait(), meaning that we also take the wq->lock.
>
> In short, wq->lock is a sync. mechanism in this case. The scheme is as follows:
>
> our side:
>
> [ finish_wait() ]
>
> lock(wq->lock);

But we can skip lock(wq->lock), afaics.

Without rmb(), test_bit() can be re-ordered with list_empty_careful()
in finish_wait() and even with __set_task_state(TASK_RUNNING).

> p.s. if the explanation is vague or heh even wrong, it's definitely
> due to the lack of sleep ;-))

The same on my side ;)

Oleg.