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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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