linux-kernel - Re: [PATCH tip/core/rcu 12/15] rcu: Remove redundant memory barrier from __call

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Message-ID: <20120831184055.GL4259@jtriplet-mobl1>
Date:	Fri, 31 Aug 2012 11:40:56 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, sbw@....edu, patches@...aro.org,
	"Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH tip/core/rcu 12/15] rcu: Remove redundant memory barrier
 from __call_rcu()

On Fri, Aug 31, 2012 at 11:30:35AM -0700, Josh Triplett wrote:
> On Thu, Aug 30, 2012 at 11:56:25AM -0700, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paul.mckenney@...aro.org>
> > 
> > The first memory barrier in __call_rcu() is supposed to order any
> > updates done beforehand by the caller against the actual queuing
> > of the callback.  However, the second memory barrier (which is intended
> > to order incrementing the queue lengths before queuing the callback)
> > is also between the caller's updates and the queuing of the callback.
> > The second memory barrier can therefore serve both purposes.
> > 
> > This commit therefore removes the first memory barrier.
> 
> I don't see any such second memory barrier in __call_rcu(), at least not
> in current master.  Right after this smp_mb(), __call_rcu() enqueues the
> callback and increments the queue length.
> 
> Did you add a second memory barrier in some other patch that hasn't made
> it upstream yet?  If so, could you note that patch dependency explicitly
> in the commit message?

Argh, nevermind.  Looked at the wrong branch, not master.  Looking at
master, I do indeed see the second smp_mb().

Reviewed-by: Josh Triplett <josh@...htriplett.org>
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