Re: wake_up_process implied memory barrier clarification

Paul E. McKenney
2015-08-31T20:40:57Z

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?

Thanx, Paul