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

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Message-ID: <20150917170111.GA29215@redhat.com>
Date:	Thu, 17 Sep 2015 19:01:11 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 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.

Oleg.

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