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    SubjectRe: linux-next: percpu tree build warning

    * Tejun Heo <tj@kernel.org> wrote:

    > 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);

    At least to me a typo like this would stick out like a sore thumb during
    review.

    I'd recognize &reg1 as a stack local variable immediately, and when i
    see it being used in this_cpu_inc() i'd go 'huh' immediately.

    OTOH, the two examples of confusion i gave you in my previous mail would
    be far less obvious. The 'visual distance' to a percpu variable
    definition is greater (it's at least file scope in 95% of the cases), so
    i wouldnt be able to 'see' which the percpu variables are, from a code
    context.

    Anyway, YMMV.

    Thanks,

    Ingo


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