linux-kernel - Re: wake_up_process implied memory barrier clarification

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Message-ID: <20150831203739.GX4029@linux.vnet.ibm.com>
Date:	Mon, 31 Aug 2015 13:37:39 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.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 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

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