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Date: Tue, 1 Sep 2015 11:41:27 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.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
Message-ID: <20150901094127.GA31368@redhat.com>
References: <20150827124334.GY16853@twins.programming.kicks-ass.net> <20150827131444.GE27052@dhcp22.suse.cz> <20150827182654.GA12191@redhat.com> <20150828145121.GG5301@dhcp22.suse.cz> <20150828160637.GA4393@redhat.com> <20150829092514.GA3240@fixme-laptop.cn.ibm.com> <20150829142707.GA19263@redhat.com> <20150831003719.GC924@fixme-laptop.cn.ibm.com> <20150831183335.GA26333@redhat.com> <20150831203739.GX4029@linux.vnet.ibm.com>
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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.