linux-kernel - Re: [PATCH v2 6/9] rcu: use __this_cpu_read helper instead of per_cpu_ptr(p, raw_smp_processor

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Message-ID: <0000013ad12afc1c-ceb72c4a-b0f2-4e79-8b09-9d7b716ce06b-000000@email.amazonses.com>
Date:	Mon, 5 Nov 2012 15:23:06 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Shan Wei <shanwei88@...il.com>, dipankar@...ibm.com,
	Kernel-Maillist <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 6/9] rcu: use __this_cpu_read helper instead of
 per_cpu_ptr(p, raw_smp_processor_id())

On Sat, 3 Nov 2012, Paul E. McKenney wrote:

> OK, I do understand why it happens to work.  My question is instead why
> it is considered a good idea.  After all, it is the ->rda field that is
> marked __percpu, not the ->mynode field.  So in the interest of
> mechanical checking and general readability, it seems to me that it
> would be way better to apply __this_cpu_read() to rsp->rda rather than
> to rsp->rda->mynode.

mynode is part of the structure reached via rda.

Use on rsp->rda does not work since the offset of mynode must be added to
rda before a fetch related to the current cpus per cpu address can be
done.

this_cpu_ptr relocates and address. this_cpu_read() relocates the address
and performs the fetch. If you want to operate on rda then you can only
use this_cpu_ptr. this_cpu_read() saves you more instructions since it can
do the relocation and the fetch in one instruction.

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