linux-kernel - Re: linux-next: percpu tree build warning

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Date:	Fri, 27 Nov 2009 14:57:59 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Rusty Russell <rusty@...tcorp.com.au>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Fr??d??ric Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Lameter <cl@...ux-foundation.org>,
	linux-next@...r.kernel.org, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: linux-next: percpu tree build warning

Hello,

11/27/2009 02:41 PM, Ingo Molnar wrote:
> But allowing &dr7 is outright dangerous - and not particularly clean 
> either.
> 
> Nothing tells us that it's a percpu variable and it blends into the 
> regular namespace while most of the operators on it are special 
> (__get_cpu_var(), per_cpu(), __this_cpu(), etc.).
> 
> What if someone writes &dr7 in preemptible code? It's dangerous to do it 
> and a quick review wont catch the mistake. Seeing &per_cpu_dr7 in 
> clearly preemptible code does raise alarms on the other hand.
>
> So i think it should be valid to take the address of it and unify the 
> static and dynamic percpu space ... if it's prefixed properly: what's 
> wrong with &per_cpu_dr7?

DEFINE_PER_CPU(unsigned long, reg0);
DEFINE_PER_CPU(unsigned long, reg1);

static void my_fn(void)
{
	unsigned long reg0 = per_cpu_var(reg0);
	unsigned long reg1 = per_cpu_var(reg1);
	unsigned long *p = &per_cpu_var(reg0);

	// blah blah

	if (some cond)
		p = &reg1;	// oops meant &per_cpu_var(reg1)

	// blah blah

	this_cpu_inc(p);
}

It's more dangerous to depend on the pseudo namespace created by
prefixing.  Let's add __percpu sparse annotations.  It will be more
flexible and safer.

Thanks.

-- 
tejun
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