linux-kernel - Re: [PATCH tip/core/rcu 02/14] documentation: Fix control dependency and identical stores

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Message-ID: <20160225140816.GK3522@linux.vnet.ibm.com>
Date:	Thu, 25 Feb 2016 06:08:16 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jianyu Zhan <nasa4836@...il.com>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Lai Jiangshan <jiangshanlai@...il.com>, dipankar@...ibm.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	josh@...htriplett.org, Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	rostedt <rostedt@...dmis.org>,
	David Howells <dhowells@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>, dvhart@...ux.intel.com,
	fweisbec@...il.com, Oleg Nesterov <oleg@...hat.com>,
	bobby prani <bobby.prani@...il.com>
Subject: Re: [PATCH tip/core/rcu 02/14] documentation: Fix control dependency
 and identical stores

On Thu, Feb 25, 2016 at 02:41:07PM +0800, Jianyu Zhan wrote:
> On Thu, Feb 25, 2016 at 5:40 AM, Paul E. McKenney
> <paulmck@...ux.vnet.ibm.com> wrote:
> > So ordering between the read from "a" and the write to "b" is still
> > preserved.  The reason this works is that the smp_mb() does all the
> > ordering, so the fact that the control dependency has been eliminated
> > is irrelevant.
> 
> 
> Thanks, Paul,  nice clarification.
> 
> I thinks this example qualifies as an good example to demonstrate the subtle
> scope of effect of these two level barriers.
> 
> A vivid example is always better for understanding instead of learning
> "barrier() is compiler-level
> barrier, and smp_*mb() is processor-level barrier" by rote ,
> especially for the new comers to memory-barriers.txt. ;-)

Glad you like it.  ;-)

							Thanx, Paul