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

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Message-ID: <50923956.5090206@gmail.com>
Date:	Thu, 01 Nov 2012 16:56:54 +0800
From:	Shan Wei <shanwei88@...il.com>
To:	Christoph Lameter <cl@...ux.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

Christoph Lameter said, at 2012/11/1 1:35:
> 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.

this_cpu_read
|-----_this_cpu_generic_read

#define _this_cpu_generic_read(pcp)                                     \
({      typeof(pcp) ret__;                                              \
        preempt_disable();                                              \
        ret__ = *this_cpu_ptr(&(pcp));                                  \
        preempt_enable();                                               \
        ret__;                                                          \
})


this_cpu_read operations locate per-cpu variable with preemption safe， not
disable interrupts. why is it atomic vs interrupts?

I have no idea whether we need to disable preemption for this code?
At least, xfrm code run well with per_cpu_ptr which don't disable preemption.

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