linux-kernel - Re: [PATCH v2] ipc/mqueue: remove STATE

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Message-ID: <1430240361.2004.13.camel@stgolabs.net>
Date:	Tue, 28 Apr 2015 09:59:21 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Manfred Spraul <manfred@...orfullife.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Darren Hart <darren@...art.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	fredrik.markstrom@...driver.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2] ipc/mqueue: remove STATE_PENDING

On Tue, 2015-04-28 at 18:43 +0200, Peter Zijlstra wrote:
> Well, if you can 'guarantee' the cmpxchg will not fail, you can then
> rely on the fact that cmpxchg implies a full barrier, which would
> obviate the need for the wmb.

Yes, assuming it implies barriers on both sides. And we could obviously
remove the need for pairing. With wake_q being local to wq_sleep() I
cannot see duplicate tasks trying to add themselves in the list. Failed
cmpxchg should only occur when users start misusing the wake_q.

Manfred, do you have any objections to this? Perhaps I've missed the
real purpose of the barriers.

Thanks,
Davidlohr


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