linux-kernel - Re: wake_up_process implied memory barrier clarification

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Message-ID: <20150901040336.GA4029@linux.vnet.ibm.com>
Date:	Mon, 31 Aug 2015 21:03:36 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Boqun Feng <boqun.feng@...il.com>
Cc:	Oleg Nesterov <oleg@...hat.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 Tue, Sep 01, 2015 at 11:40:14AM +0800, Boqun Feng wrote:
> Hi Paul,
> 
> On Mon, Aug 31, 2015 at 01:37:39PM -0700, 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?
> 
> If there is no sleep and no wakeup, it means only CONDITION changed.
> Either CONDITION is a single variable or it should maintains internal
> ordering guarantee itself. And there is no need for barriers, because
> there is only one shared resource we are talking about, right?

I could imagine all sorts of combinations, which is why I would like
to see a code snippet showing exactly what Oleg is talking about.  ;-)

							Thanx, Paul

> But I'm still a little confused at Oleg's words:
> 
> "What is really important is that we have a barrier before we _read_ the
> task state."
> 
> I read is as "What is really important is that we have a barrier before
> we _read_ the task state and _after_ we write the CONDITION", if I don't
> misunderstand Oleg, this means a STORE-barrier-LOAD sequence, which IIUC
> can't pair with anything.
> 
> So, there might be some tricky barrier usage here?
> 
> Regards,
> Boqun


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