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

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Date:	Thu, 9 Apr 2009 13:47:14 +0400
From:	Ivan Kokshaysky <ink@...assic.park.msu.ru>
To:	Tejun Heo <tj@...nel.org>
Cc:	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Ingo Molnar <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,
	heiko.carstens@...ibm.com
Subject: Re: [GIT RFC] percpu: use dynamic percpu allocator as the default
	percpu allocator

On Wed, Apr 08, 2009 at 01:18:23PM -0700, Tejun Heo wrote:
> > #define DEFINE_PER_CPU_SECTION(type, name, section)			\
> > 	__attribute__((__section__(".garbage"), __unused__))		\
> > 		char __dummy__##name;					\
> > 	__attribute__((__section__(PER_CPU_BASE_SECTION section)))	\
> > 		__weak __typeof__(type) per_cpu__##name
> 
> Nice.  I think .discard would suit better for the dummy variable tho
> (x86 discards the section and it seems to be more common name with
> ldscript using /DISCARD/ identifier for things to throw away).  Also,

Indeed, .discard sounds better.

> it seems like if two different files happen to use static percpu
> variables with the same name, they would end up sharing it, right?
> That looks a tad bit dangerous.

True. One of possible solutions is to add another dummy variable,
like this:

#define DEFINE_PER_CPU_SECTION(type, name, section)			\
	__attribute__((__section__(".discard"), __unused__))		\
		char __dummy__##name;					\
+	__attribute__((__section__(".discard")))			\
+		char __per_cpu_multiple_def__##name;			\
	__attribute__((__section__(PER_CPU_BASE_SECTION section)))	\
		__weak __typeof__(type) per_cpu__##name

so in that situation we'll get a link failure.

Ivan.
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