linux-kernel - Re: [PATCH 3/9] net: xfrm: use this_cpu

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Message-ID: <0000013ab7e4a640-60bd5b38-a1fc-4730-b918-4109211ffea0-000000@email.amazonses.com>
Date:	Wed, 31 Oct 2012 17:35:46 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Shan Wei <shanwei88@...il.com>
cc:	steffen.klassert@...unet.com, David Miller <davem@...emloft.net>,
	NetDev <netdev@...r.kernel.org>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Kernel-Maillist <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/9] net: xfrm: use this_cpu_ptr per-cpu helper

On Wed, 31 Oct 2012, Shan Wei wrote:

> -
>  	list_for_each_entry(pos, &ipcomp_tfms_list, list) {
>  		struct crypto_comp *tfm;
>
>  		tfms = pos->tfms;
> -		tfm = *per_cpu_ptr(tfms, cpu);
> +
> +		/* This can be any valid CPU ID so we don't need locking. */
> +		tfm = *this_cpu_ptr(tfms);

It would be better to use

	this_cpu_read(tfms)

since that would also make it atomic vs interrupts. The above code (both
original and modified) could determine a pointer to a per cpu structure
and then take an interrupt which would move the task. On return we would
be accessing the per cpu variable of another processor.

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