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

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Message-ID: <20131210184920.GA4208@linux.vnet.ibm.com>
Date:	Tue, 10 Dec 2013 10:49:20 -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 06:43:45PM +0100, Peter Zijlstra wrote:
> 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?

Yep!

> > 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 :-)

Good eyes!  I changed this to:

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

Is that better?

> > 		*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 :-)

Indeed they can!  ;-)

> > 	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. :-)

;-) ;-) ;-)

							Thanx, Paul

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