linux-kernel - Re: [PATCH] Documentation: Remove misleading examples of the barriers in wake

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Date:	Fri, 18 Sep 2015 08:49:56 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Boqun Feng <boqun.feng@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	Jonathan Corbet <corbet@....net>,
	Michal Hocko <mhocko@...nel.org>,
	David Howells <dhowells@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Will Deacon <will.deacon@....com>
Subject: Re: [PATCH] Documentation: Remove misleading examples of the
 barriers in wake_*()

On Thu, Sep 17, 2015 at 07:01:11PM +0200, Oleg Nesterov wrote:
> On 09/17, Peter Zijlstra wrote:
> >
> > Included in it are some of the details on this subject, because a wakeup
> > has two prior states that are of importance, the tasks own prior state
> > and the wakeup state, both should be considered in the 'program order'
> > flow.
> 
> Great. Just one question,
> 
> > + *   BLOCKING -- aka. SLEEP + WAKEUP
> > + *
> > + * For blocking things are a little more interesting, because when we dequeue
> > + * the task, we don't need to acquire the old rq lock in order to migrate it.
> > + *
> > + * Say CPU0 does a wait_event() and CPU1 does the wake() and migrates the task
> > + * to CPU2 (the most complex example):
> > + *
> > + *   CPU0 (schedule)  CPU1 (try_to_wake_up) CPU2 (sched_ttwu_pending)
> > + *
> > + *   X->state = UNINTERRUPTIBLE
> > + *   MB
> > + *   if (cond)
> > + *     break
> > + *                    cond = true
> > + *
> > + *   WMB              WMB (aka smp_mb__before_spinlock)
> 
> Yes, both CPU's do WMB-aka-smp_mb__before_spinlock...
> 
> But afaics in this particular case we do not really need them?
> So perhaps we should not even mention them?
> 
> Because (if I am right) this can confuse the reader who will try
> to understand how/where do we rely on these barriers.

Good point. Initially I put all barriers in, but now that we've figured
out which are important (the text is correct, right? please double
check) we can remove the rest.
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