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

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Message-ID: <20140617185510.GM4669@linux.vnet.ibm.com>
Date:	Tue, 17 Jun 2014 11:55:10 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Christoph Lameter <cl@...two.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, David Howells <dhowells@...hat.com>,
	linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH RFC] percpu: add data dependency barrier in percpu
 accessors and operations

On Tue, Jun 17, 2014 at 01:47:01PM -0500, Christoph Lameter wrote:
> 
> On Tue, 17 Jun 2014, Linus Torvalds wrote:
> 
> > On Jun 17, 2014 6:28 AM, "Christoph Lameter" <cl@...two.org> wrote:
> > >
> > > Even alpha maintains the illusion of changes becoming visible in the
> > > proper order for the currently executing thread. No barriers are needed.
> >
> > No Christoph, alpha really doesn't. Barriers needed.
> 
> Everythig breaks if a single hardware execution thread can no longer
> observe a consistent view of the world of the data it modifies.
> 
> We are not talking about synchronization here between multiple hardware
> threads. This is simply code running on a single hardware thread and all
> architectures maintain the uniprocessor illusion there.

We are talking about one CPU initializing all CPUs' portions of
dynamically allocated per-CPU memory, so there really is more than
one CPU involved.

							Thanx, Paul

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