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Date: Mon, 31 Aug 2015 13:37:39 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Boqun Feng <boqun.feng@gmail.com>, Michal Hocko <mhocko@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        David Howells <dhowells@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Jonathan Corbet <corbet@lwn.net>
Subject: Re: wake_up_process implied memory barrier clarification
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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