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Subject: RE: [PATCH 3/9] net: xfrm: use this_cpu_ptr per-cpu helper
Date: Thu, 1 Nov 2012 12:15:50 -0000
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thread-topic: [PATCH 3/9] net: xfrm: use this_cpu_ptr per-cpu helper
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From: "David Laight" <David.Laight@ACULAB.COM>
To: "Shan Wei" <shanwei88@gmail.com>, "Christoph Lameter" <cl@linux.com>
Cc: <steffen.klassert@secunet.com>, "David Miller" <davem@davemloft.net>,
        "NetDev" <netdev@vger.kernel.org>,
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> 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?

Hmmm...  what effect do those preemt_dis/enable() actually have?
Since a pre-empt can happen either side of them, the value
the caller sees can be for the wrong cpu anyway.

The only time I could see them being necessary is if
*this_cpu_ptr() itself needs mutex protection in order to
function correctly - and that is likely to be port specific.
On i386/amd64 where (I guess) it is an access offset by fs/gs
this isn't necessary and just wastes cpu cycles.

If the caller cares which cpu the value comes from (eg to
increment a counter) then the caller would need to disable
pre-emption across the whole operation.

	David

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