Re: wake_up_process implied memory barrier clarification

Paul E. McKenney
2015-09-01T04:03:43Z

On Tue, Sep 01, 2015 at 11:40:14AM +0800, Boqun Feng wrote:
> Hi Paul,
>
> On Mon, Aug 31, 2015 at 01:37:39PM -0700, Paul E. McKenney wrote:
> > On Mon, Aug 31, 2015 at 08:33:35PM +0200, Oleg Nesterov wrote:
> > > On 08/31, Boqun Feng wrote:
> > > >
> > > > Fair enough, I went too far. How about just a single paragraph saying
> > > > that:
> > > >
> > > > The wake_up(), wait_event() and their friends have proper barriers in
> > > > them, but these implicity barriers are only for the correctness for
> > > > sleep and wakeup. So don't rely on these barriers for things that are
> > > > neither wait-conditons nor task states.
> > > >
> > > > Is that OK to you?
> > >
> > > Ask Paul ;) but personally I agree.
> > >
> > > To me, the only thing a user should know about wake_up/try_to_wake_up
> > > and barriers is that you do not need another barrier between setting
> > > condition and waking up.
> >
> > Sounds like an excellent idea in general. But could you please show me
> > a short code snippet illustrating where you don't need the additional
> > barrier, even if the fastpaths are taken so that there is no sleep and
> > no wakeup?
>
> If there is no sleep and no wakeup, it means only CONDITION changed.
> Either CONDITION is a single variable or it should maintains internal
> ordering guarantee itself. And there is no need for barriers, because
> there is only one shared resource we are talking about, right?

I could imagine all sorts of combinations, which is why I would like
to see a code snippet showing exactly what Oleg is talking about. ;-)

Thanx, Paul

> But I'm still a little confused at Oleg's words:
>
> "What is really important is that we have a barrier before we _read_ the
> task state."
>
> I read is as "What is really important is that we have a barrier before
> we _read_ the task state and _after_ we write the CONDITION", if I don't
> misunderstand Oleg, this means a STORE-barrier-LOAD sequence, which IIUC
> can't pair with anything.
>
> So, there might be some tricky barrier usage here?
>
> Regards,
> Boqun