Re: linux-next: percpu tree build warning

Tejun Heo
2009-11-27T05:58:16Z

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