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

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Message-ID: <20140617195634.GQ4669@linux.vnet.ibm.com>
Date:	Tue, 17 Jun 2014 12:56:35 -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 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.

Given runtime dynamic allocation of per-CPU memory, you still need
proper synchronization.  And yes, on non-Alpha CPUs, the dependency
ordering through any pointer suffices on the use side.  The thing
doing allocation and initialization will still need memory barriers,
of course.

> 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.

During operation that does not involve cross-CPU accesses, agreed.

							Thanx, Paul

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