Re: wake_up_process implied memory barrier clarification

From	Oleg Nesterov <oleg@redhat.com>
Newsgroups	linux.kernel
Subject	Re: wake_up_process implied memory barrier clarification
Date	2015-09-01 11:50 +0200
Message-ID	<q3Qmu-2NS-9@gated-at.bofh.it> (permalink)
References	(5 earlier) <q2KCt-65w-1@gated-at.bofh.it> <q2Psu-4AG-33@gated-at.bofh.it> <q3liF-8sy-5@gated-at.bofh.it> <q3C9Q-7wi-7@gated-at.bofh.it> <q3EbE-1Wt-11@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On 08/31, 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?

I guess I wasn't clear... All I tried to say is that

	CONDITION = 1;
	wake_up_process();

does not need any _additional_ barrier in between.

I mentioned this because afaics people are often unsure if this is true
or not, and to some degree this question initiated this discussion.

Oleg.

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Thread

wake_up_process implied memory barrier clarification Michal Hocko <mhocko@kernel.org> - 2015-08-27 14:30 +0200
  Re: wake_up_process implied memory barrier clarification Peter Zijlstra <peterz@infradead.org> - 2015-08-27 14:50 +0200
    Re: wake_up_process implied memory barrier clarification Michal Hocko <mhocko@kernel.org> - 2015-08-27 15:20 +0200
      Re: wake_up_process implied memory barrier clarification Oleg Nesterov <oleg@redhat.com> - 2015-08-27 20:30 +0200
        Re: wake_up_process implied memory barrier clarification Michal Hocko <mhocko@kernel.org> - 2015-08-28 17:00 +0200
          Re: wake_up_process implied memory barrier clarification Oleg Nesterov <oleg@redhat.com> - 2015-08-28 18:10 +0200
            Re: wake_up_process implied memory barrier clarification Boqun Feng <boqun.feng@gmail.com> - 2015-08-29 11:30 +0200
              Re: wake_up_process implied memory barrier clarification Oleg Nesterov <oleg@redhat.com> - 2015-08-29 16:40 +0200
                Re: wake_up_process implied memory barrier clarification Boqun Feng <boqun.feng@gmail.com> - 2015-08-31 02:40 +0200
                Re: wake_up_process implied memory barrier clarification Oleg Nesterov <oleg@redhat.com> - 2015-08-31 20:40 +0200
                Re: wake_up_process implied memory barrier clarification "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-08-31 22:50 +0200
                Re: wake_up_process implied memory barrier clarification Boqun Feng <boqun.feng@gmail.com> - 2015-09-01 05:50 +0200
                Re: wake_up_process implied memory barrier clarification "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-09-01 06:10 +0200
                Re: wake_up_process implied memory barrier clarification Oleg Nesterov <oleg@redhat.com> - 2015-09-01 12:10 +0200
                Re: wake_up_process implied memory barrier clarification Boqun Feng <boqun.feng@gmail.com> - 2015-09-01 17:00 +0200
                Re: wake_up_process implied memory barrier clarification Oleg Nesterov <oleg@redhat.com> - 2015-09-01 18:50 +0200
                Re: wake_up_process implied memory barrier clarification Boqun Feng <boqun.feng@gmail.com> - 2015-09-02 03:20 +0200
                Re: wake_up_process implied memory barrier clarification Oleg Nesterov <oleg@redhat.com> - 2015-09-01 11:50 +0200

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