Message-ID: <4A3993A2.9070109@kernel.org>
Date: Thu, 18 Jun 2009 10:08:50 +0900
From: Tejun Heo <tj@kernel.org>
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To: Christoph Lameter <cl@linux-foundation.org>
CC: linux-kernel@vger.kernel.org, David Howells <dhowells@redhat.com>,
       Ingo Molnar <mingo@elte.hu>, Rusty Russell <rusty@rustcorp.com.au>,
       Eric Dumazet <dada1@cosmosbay.com>, davem@davemloft.net
Subject: Re: [this_cpu_xx 01/11] Introduce this_cpu_ptr() and generic this_cpu_*
 operations
References: <20090605191819.376530498@gentwo.org> <20090605191850.441840884@gentwo.org> <4A38A719.2030709@kernel.org> <alpine.DEB.1.10.0906171343370.30060@gentwo.org>
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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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