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

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Date:	Fri, 20 Jun 2014 10:23:57 -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
Subject: Re: [PATCH RFC] percpu: add data dependency barrier in percpu
 accessors and operations

On Thu, 19 Jun 2014, Tejun Heo wrote:

> Again, data dependency barrier is noop in all in-use archs.

A barrier limits what the compiler can do.

> > Remote write events are contrary to that design and are exceedingly rare.
> > An IPI is justifiable for such a rare event. At least in my use cases I
> > have always found that to be sufficient. Well, I designed the data
> > structures in a way that made this possible because of the design criteria
> > that did not allow me remote write access to other processors per cpu
> > data.
>
> You're repeatedly getting wayside in the discussion.  What are you
> suggesting?  Sending IPIs on each percpu allocation?

No this is about sending an IPI if you want to modify the percpu data of
another process. There was a mentionig of code that modifies the per cpu
data of another processor?

> Again, I'm leaning towards just clarifying the init write ownership to
> the allocating CPU as that seems the most straight forward way to deal
> with it, but please stop brining up the raw performance thing.  Nobody
> is doing anything to that.  It's not relevant in the discussion.

Ok sounds good.

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