linux-kernel - Re: [PATCH RFC] percpu: add data dependency barrier in percpu accessors and operations

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Message-ID: <alpine.DEB.2.11.1406171125340.20610@gentwo.org>
Date:	Tue, 17 Jun 2014 11:28:19 -0500 (CDT)
From:	Christoph Lameter <cl@...two.org>
To:	Tejun Heo <tj@...nel.org>
cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	David Howells <dhowells@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH RFC] percpu: add data dependency barrier in percpu
 accessors and operations

On Tue, 17 Jun 2014, Tejun Heo wrote:

> On Tue, Jun 17, 2014 at 12:00:40PM -0400, Tejun Heo wrote:
> > Hello, Christoph.
> >
> > On Tue, Jun 17, 2014 at 10:56:10AM -0500, Christoph Lameter wrote:
> > > Uhhh no. The percpu stuff and the associated per cpu atomics are to be
> > > used for stuff that is per cpu specific and runs at the fastest speed
> > > doable at that level. Introducing implicit barriers is not that good an
> > > idea.
> >
> > Hmmm?  Read barriers are noops on all archs except for alpha and
>
> Oops, data dependency barriers, not read barrier.

Even alpha maintains the illusion of changes becoming visible in the
proper order for the currently executing thread. No barriers are needed.


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