linux-kernel - wake_up_process implied memory barrier clarification

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Message-ID: <20150827122727.GC27052@dhcp22.suse.cz>
Date:	Thu, 27 Aug 2015 14:27:27 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	David Howells <dhowells@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: wake_up_process implied memory barrier clarification

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?
-- 
Michal Hocko
SUSE Labs
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