linux-kernel - Re: futex atomic vs ordering constraints

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Message-ID: <20150901163812.GN16853@twins.programming.kicks-ass.net>
Date:	Tue, 1 Sep 2015 18:38:12 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Davidlohr Bueso <dave@...olabs.net>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>, mtk.manpages@...il.com,
	dvhart@...radead.org, Vineet.Gupta1@...opsys.com,
	ralf@...ux-mips.org, ddaney@...iumnetworks.com,
	Will Deacon <will.deacon@....com>, linux-kernel@...r.kernel.org
Subject: Re: futex atomic vs ordering constraints

On Fri, Aug 28, 2015 at 06:33:06PM -0700, Davidlohr Bueso wrote:
> On Wed, 2015-08-26 at 20:16 +0200, Peter Zijlstra wrote:
> > Of course, if anything else prior to futex_atomic_op_inuser() implies an
> > (RCsc) RELEASE or stronger the primitive can do without providing
> > anything itself.
> > 
> > This turns out to be the case, a successful get_futex_key() implies a
> > full memory barrier; recent: 1d0dcb3ad9d3 ("futex: Implement lockless
> > wakeups").
> 
> Hmm while it is certainly true that get_futex_key() implies a full
> barrier, I don't see why you're referring to the recent wake_q stuff;

D'oh, because I'm a sheep or so. I meant:

  b0c29f79ecea (futexes: Avoid taking the hb->lock if there's nothing to wake up)
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