linux-kernel - Re: [PATCH v5 tip/core/locking 5/7] Documentation/memory-barriers.txt: Downgrade UNLOCK+LOCK

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Date:	Tue, 10 Dec 2013 09:43:20 -0800
From:	Josh Triplett <josh@...htriplett.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	niv@...ibm.com, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, darren@...art.com,
	fweisbec@...il.com, sbw@....edu, Ingo Molnar <mingo@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Will Deacon <will.deacon@....com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Waiman Long <waiman.long@...com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	"H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH v5 tip/core/locking 5/7]
 Documentation/memory-barriers.txt: Downgrade UNLOCK+LOCK

On Tue, Dec 10, 2013 at 06:25:28PM +0100, Peter Zijlstra wrote:
> On Tue, Dec 10, 2013 at 09:12:47AM -0800, Paul E. McKenney wrote:
> > > The way I read the above it says that you need
> > > smp_mb__after_unlock_lock() when the UNLOCK and LOCK are on the same
> > > variable. That doesn't make sense, I thought that was the one case we
> > > all agreed on it would indeed be a full barrier without extra trickery.
> > 
> > On x86, sure, but smp_mb__after_unlock_lock() is nothingness on x86
> > anyway.  Other architectures might benefit from requiring that the
> > smp_mb__after_unlock_lock() be used in this case.
> 
> Confused, UNLOCK X, LOCK X, must always be fully serializing. That's the
> entire purpose of the thing.
> 
> The only place you can go play games (and clearly we are going there) is
> when the UNLOCK and LOCK are on different variables.

That would certainly be a good assumption to preserve, and it would
eliminate most of the need for smp_mb__after_unlock_lock().

- Josh Triplett
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