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

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Message-ID: <20131210174345.GF13532@twins.programming.kicks-ass.net>
Date:	Tue, 10 Dec 2013 18:43:45 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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 09:12:47AM -0800, Paul E. McKenney wrote:
> Good point -- the UNLOCK and LOCK are guaranteed to be ordered only
> if they both operate on the same lock variable.  OK, I will make the
> example use different lock variables and show the different outcomes.
> How about the following?
> 
> 	If it is necessary for an UNLOCK-LOCK pair to
> 	produce a full barrier, the LOCK can be followed by an
> 	smp_mb__after_unlock_lock() invocation.  This will produce a
> 	full barrier if either (a) the UNLOCK and the LOCK are executed
> 	by the same CPU or task, or (b) the UNLOCK and LOCK act on the
> 	same lock variable.  

So you're still requiring smp_mb__after_unlock_lock() even if they're on
the same variable?

> The smp_mb__after_unlock_lock() primitive is
> 	free on many architectures.  Without smp_mb__after_unlock_lock(),
> 	the UNLOCK and LOCK can cross:

Contradicted below :-)

> 		*A = a;
> 		UNLOCK M
> 		LOCK N
> 		*B = b;
> 
> 	could occur as:
> 
> 		LOCK N, STORE *B, STORE *A, UNLOCK M
> 
> 	With smp_mb__after_unlock_lock(), they cannot, so that:
> 
> 		*A = a;
> 		UNLOCK M
> 		LOCK N
> 		smp_mb__after_unlock_lock();
> 		*B = b;
> 
> 	will always occur as either of the following:
> 
> 		STORE *A, UNLOCK, LOCK, STORE *B
> 		STORE *A, LOCK, UNLOCK, STORE *B

See, UNLOCK and LOCK can still cross :-)

> 	If the UNLOCK and LOCK were instead both operating on the same
> 	lock variable, only the first of these two alternatives can occur.

Agreed.

Sorry for being a pedant. :-)
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