linux-kernel - Re: [this_cpu_xx 01/11] Introduce this_cpu_ptr() and generic this_cpu

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Date:	Thu, 18 Jun 2009 10:08:50 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Christoph Lameter <cl@...ux-foundation.org>
CC:	linux-kernel@...r.kernel.org, David Howells <dhowells@...hat.com>,
	Ingo Molnar <mingo@...e.hu>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Eric Dumazet <dada1@...mosbay.com>, davem@...emloft.net
Subject: Re: [this_cpu_xx 01/11] Introduce this_cpu_ptr() and generic this_cpu_*
 operations

Hello,

Christoph Lameter wrote:
> On Wed, 17 Jun 2009, Tejun Heo wrote:
>> Please note that this can also happen between addition or other
>> modifying ops and cause incorrect result.
> 
> Per cpu operations are only safe for the current processor. One issue
> there may be that the store after rescheduling may not occur to the
> current processors per cpu instance but the prior cpu. At that point
> another thread may be running on the prior cpu and be disturbed like you
> point out. So it needs a preempt disable there too.

Yeap, to summarize, the problem is that the address determination and
the actual memory write aren't atomic with respect to preeamption.

>> Also, these macros depricate percpu_OP() macros, right?
> 
> They are different. percpu_OP() macros require a percpu variable name
> to be passed.
> 
> this_cpu_* macros require a reference to a variable in a
> structure allocated with the new per cpu allocator.
> 
> It is possible to simply pass the full variable name of a percpu variable
> to this_cpu_* macros. See the patch of the vm statistics handling.
> 
> It uses
> 
> 	per_cpu_var(per_cpu_name_without_prefix)
> 
> to generate the full name.

Yeap, I guess it's about time to ressurect Rusty's drop-per_cpu_
prefix patch; then, we can truly handle static and dynamic variables
in the same manner.

Thanks.

-- 
tejun
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