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 09:49:09 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, 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.

>From the viewpoint of some CPU holding the lock, yes.  If some CPU does
not hold the lock, the guarantee is a choice.

> The only place you can go play games (and clearly we are going there) is
> when the UNLOCK and LOCK are on different variables.

I agree that if UNLOCK and LOCK are on different variables, we do not
need to guarantee ordering of the two critical sections.  But if they
are on the same variable, why would it be different from the viewpoint
of some CPU not holding the lock?

							Thanx, Paul

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