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

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Date:	Mon, 21 Sep 2015 19:46:11 +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 09/18, Peter Zijlstra wrote:
>
> the text is correct, right?

Yes, it looks good to me and helpful.

But damn. I forgot why exactly try_to_wake_up() needs rmb() after
->on_cpu check... It looks reasonable in any case, but I do not
see any strong reason immediately.

Say,

	p->sched_contributes_to_load = !!task_contributes_to_load(p);
	p->state = TASK_WAKING;

we can actually do this before "while (p->on_cpu)", afaics. However
we must not do this before the previous p->on_rq check.

So perhaps this rmb() helps to ensure task_contributes_to_load() can't
happen before p->on_rq check...

As for "p->state = TASK_WAKING" we have the control dependency in both
cases. But the modern fashion suggests to use _CTRL(). Although cpu_relax()
should imply barrier(), but afaik this is not documented.

In short, I got lost ;) Now I don't even understand why we do not need
another rmb() between p->on_rq and p->on_cpu. Suppose a thread T does

	set_current_state(...);
	schedule();

it can be preempted in between, after that we have "on_rq && !on_cpu".
Then it gets CPU again and calls schedule() which clears on_rq.

What guarantees that if ttwu() sees on_rq == 0 cleared by schedule()
then it can _not_ still see the old value of on_cpu == 0?

Oleg.

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