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

From: Tejun Heo <tj@kernel.org>
To: Christoph Lameter <cl@gentwo.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Rusty Russell <rusty@rustcorp.com.au>,
	Andrew Morton <akpm@linux-foundation.org>,
	David Howells <dhowells@redhat.com>,
	linux-kernel@vger.kernel.org, Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH RFC] percpu: add data dependency barrier in percpu accessors and operations
Date: Tue, 17 Jun 2014 15:47:00 -0400	[thread overview]
Message-ID: <20140617194700.GP31819@htj.dyndns.org> (raw)
In-Reply-To: <alpine.DEB.2.11.1406171437570.22064@gentwo.org>

On Tue, Jun 17, 2014 at 02:39:46PM -0500, Christoph Lameter wrote:
> On Tue, 17 Jun 2014, Paul E. McKenney wrote:
> 
> > 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.
> 
> Well that only occurs on initialization before the address of the
> struct that contains the offset is available to other processors.

So does assignment of the pointer in most cases.

> During operation the percpu area functions like a single processor. And
> its designed that way to avoid synchronization issues and take full
> advantage of *no* synchronization for full speed. We compromise on that
> for statistics but that is only read access.

And during normal operation, it doesn't make any difference for
anything except alpha.  It's about making the initialization part
easier / safer.

Thanks.

-- 
tejun