linux-kernel - Re: wake_up_process implied memory barrier clarification

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Message-ID: <20150901094127.GA31368@redhat.com>
Date:	Tue, 1 Sep 2015 11:41:27 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Boqun Feng <boqun.feng@...il.com>,
	Michal Hocko <mhocko@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	David Howells <dhowells@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jonathan Corbet <corbet@....net>
Subject: Re: wake_up_process implied memory barrier clarification

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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