linux-kernel - Re: [PATCH v4] ipc/msg: Implement lockless pipelined wakeups

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Date:	Thu, 21 Jul 2016 12:23:56 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Davidlohr Bueso <dave@...olabs.net>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>, tglx@...utronix.de,
	Manfred Spraul <manfred@...orfullife.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v4] ipc/msg: Implement lockless pipelined wakeups

* Davidlohr Bueso | 2016-07-20 17:16:12 [-0700]:

>Just as with expunge_all and the E2BIG case, could you remove that explicit
>barrier (B) and just rely on wake_q_add?

Just did. So we have just a smp_rmb() on the reader side and the
comment talks about smb_wmb() and at the spot where we should have the
smb_wmb we have a comment why we don't have one :)
For my understanding: we need that smp_rmb() to ensure that everything
past that cmpxchg() is visible on all other CPUs so we don't have the
wakeup before we r_msg reads != -EAGAIN, right?

>Thanks,
>Davidlohr

Sebastian