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

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Message-ID: <20140619205137.GK4904@linux.vnet.ibm.com>
Date:	Thu, 19 Jun 2014 13:51:37 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Christoph Lameter <cl@...two.org>
Cc:	Tejun Heo <tj@...nel.org>, 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, Jun 19, 2014 at 03:42:07PM -0500, Christoph Lameter wrote:
> On Tue, 17 Jun 2014, Paul E. McKenney wrote:
> 
> > > Similar in swapon(). The percpu allocation is performed before access to
> > > the containing structure (via enable_swap_info).
> >
> > Those are indeed common use cases.  However...
> >
> > There is code where one CPU writes to another CPU's per-CPU variables.
> > One example is RCU callback offloading, where a kernel thread (which
> > might be running anywhere) dequeues a given CPU's RCU callbacks and
> > processes them.  The act of dequeuing requires write access to that
> > CPU's per-CPU rcu_data structure.  And yes, atomic operations and memory
> > barriers are of course required to make this work.
> 
> In that case special care needs to be taken to get this right. True.
> 
> I typically avoid these scenarios by sending an IPI with a pointer to the
> data structure. The modification is done by the cpu for which the per cpu
> data is local.
> 
> Maybe rewrite the code to avoid writing to other processors percpu data
> would be the right approach?

Or just keep doing what I am doing.  What exactly is the problem with it?
(Other than probably needing to clean up the cache alignment of some
of the per-CPU structures?)

							Thanx, Paul

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