linux-kernel - Re: [GIT RFC] percpu: use dynamic percpu allocator as the default percpu allocator

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Date:	Wed, 25 Mar 2009 23:14:04 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
CC:	mingo@...e.hu, rusty@...tcorp.com.au, tglx@...utronix.de,
	x86@...nel.org, linux-kernel@...r.kernel.org, hpa@...or.com,
	Paul Mundt <lethal@...ux-sh.org>, rmk@....linux.org.uk,
	starvik@...s.com, ralf@...ux-mips.org, davem@...emloft.net,
	cooloney@...nel.org, kyle@...artin.ca, matthew@....cx,
	grundler@...isc-linux.org, takata@...ux-m32r.org,
	benh@...nel.crashing.org, rth@...ddle.net,
	ink@...assic.park.msu.ru, heiko.carstens@...ibm.com
Subject: Re: [GIT RFC] percpu: use dynamic percpu allocator as the default
 percpu allocator

Martin Schwidefsky wrote:
> On Wed, 25 Mar 2009 20:51:31 +0900
> Tejun Heo <tj@...nel.org> wrote:
> 
>>> With your patches and a RELOC_HIDE version that uses the GOTENT
>>> indirection the kernel won't compile because the "X" constraint for
>>> the GOTENT access needs a symbol and there are quite a few users that
>>> pass a pointer. I do not see a simple solution for that problem yet.
>> Ah... okay.  Now I get it.  It wasn't expecting variables there.  How
>> about doing the following?
>>
>>  #define SHIFT_PERCPU_PTR(ptr, offset) (({				\
>> 	 if (__builtin_constant_p(ptr))					\
>> 		 do GOTENT trick;					\
>> 	 else								\
>> 		 RELOC_HIDE();						\
>>  }))
> 
> That doesn't work because __builtin_constant_p is false for variable
> symbols. I would need a __builtin_symbol_p but that doesn't exist.

Right, I somehow always get confused about the two.  Eh... Not easy.
I think alpha might have the same problem too.  I suppose we'll have
to make the legacy allocator available for alpha and s390 for the time
being.  I'll get to that.

Thanks.

-- 
tejun
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