linux-kernel - Re: [PATCH v2 1/1] doc: Add remote CPU access details and others to this_cpu

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Date:	Thu, 17 Jul 2014 08:50:54 -0500 (CDT)
From:	Christoph Lameter <cl@...two.org>
To:	Pranith Kumar <bobby.prani@...il.com>
cc:	Randy Dunlap <rdunlap@...radead.org>,
	"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/1] doc: Add remote CPU access details and others to
 this_cpu_ops.txt

On Wed, 16 Jul 2014, Pranith Kumar wrote:

> +You can also do the following to convert the datap offset to an address
> +
> +	struct data *p = this_cpu_ptr(&datap);
> +
> +but, passing of pointers calculated via this_cpu_ptr to other cpus is
> +unusual and should be avoided.
> +
> +Such remote accesses to per CPU data are not guaranteed to be atomic
> +anymore. You will have to use atomic_t and rely on the standard atomic
> +operations for these remote accesses to be atomic.

The use of atomic_t implies a remote write operation to a percpu area.

atomic_t needs to be avoided. If data needs to be modified from multiple
cpus then it usually does not belong into a percpu area.
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