linux-kernel - Re: wake_up_process implied memory barrier clarification

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Message-ID: <20150827124334.GY16853@twins.programming.kicks-ass.net>
Date:	Thu, 27 Aug 2015 14:43:34 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	David Howells <dhowells@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: wake_up_process implied memory barrier clarification

On Thu, Aug 27, 2015 at 02:27:27PM +0200, Michal Hocko wrote:
> Hi,
> I have just stumbled over the comment above wake_up_process which
> claims:
> "
>  * It may be assumed that this function implies a write memory barrier before
>  * changing the task state if and only if any tasks are woken up.
> "
> 
> but try_to_wake_up does smp_mb__before_spinlock and did smp_wmb
> since 04e2f1741d235 unconditionally. The comment was added when the
> smp_wmb was in place already so I am wondering whether the comment is
> wrong/misleading.
> 
> Could somebody clarify please?

Its true for wake_up(), since that bails early if the waitqueue list is
empty.

I suspect there was no exception made for wake_up_process() to simplify
the rules.
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