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

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Message-ID: <20140617160040.GE31819@htj.dyndns.org>
Date:	Tue, 17 Jun 2014 12:00:40 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Christoph Lameter <cl@...two.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,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH RFC] percpu: add data dependency barrier in percpu
 accessors and operations

Hello, Christoph.

On Tue, Jun 17, 2014 at 10:56:10AM -0500, Christoph Lameter wrote:
> Uhhh no. The percpu stuff and the associated per cpu atomics are to be
> used for stuff that is per cpu specific and runs at the fastest speed
> doable at that level. Introducing implicit barriers is not that good an
> idea.

Hmmm?  Read barriers are noops on all archs except for alpha and
percpu pointer assignments aren't exactly a high frequency operation
and I'm pretty sure we'll end up with a raw variant anyway.

> The concurrency guarantees for the per cpu operations are related to being
> interrupted or rescheduled but not for accesses from other processors.
> 
> Cpus maintain at least the appearance of operations being visible in
> sequence for code running on the same processor. Therefore no barriers are
> needed.

Huh?  We're talking about percpu *pointer* assignments not assignments
to percpu areas.

Thanks.

-- 
tejun
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