linux-kernel - Re: [PATCH v2] ipc/msg: Implement lockless pipelined wakeups

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Date:	Wed, 4 Nov 2015 10:11:40 -0800
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:	linux-kernel@...r.kernel.org, George Spelvin <linux@...izon.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Manfred Spraul <manfred@...orfullife.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2] ipc/msg: Implement lockless pipelined wakeups

On Tue, 03 Nov 2015, Sebastian Andrzej Siewior wrote:

>In pipelined_receive() (mqueue) there is the wake_q_add() with the
>implicit cmpxchg barrier. The matching barrier pairing should be in
>wq_sleep() but there is none. Why is it okay to have none there and I
>need one here?

So the pairing in mqueue is done in wq_sleep() as we unlock info->lock,
so there's an implied barrier there.

Also, considering Peter's suggestion to wrap around READ/WRITE_ONCE
for sysv queues (specially since you got rid of the volatile tag),
it seems we also need them for mqueues.

Thanks,
Davidlohr
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