Re: linux-next: percpu tree build warning

Ingo Molnar
2009-11-29T06:40:38Z

* Rusty Russell <rusty@rustcorp.com.au> wrote:

> On Fri, 27 Nov 2009 04:11:28 pm Ingo Molnar wrote:
> >
> > * Rusty Russell <rusty@rustcorp.com.au> wrote:
> >
> > > On Thu, 26 Nov 2009 12:10:58 am Ingo Molnar wrote:
> > > > While a percpu variable is defined and used in completely different
> > > > ways:
> > > >
> > > > DEFINE_PER_CPU(unsigned long, dr7);
> > > >
> > > > and is used via:
> > > >
> > > > __get_cpu_var(dr7); [[Fixed -- RR]]
> > >
> > > The entire point of Tejun's per-cpu work is that &dr7 is now valid. A
> > > per-cpu pointer as if it were allocated by the dynamic per-cpu
> > > allocator.
> > >
> > > Your arguments are fine, but out-of-date.
> >
> > But allowing &dr7 is outright dangerous - and not particularly clean
> > either.
>
> That's foolish. We can now have generic per-cpu function for counters
> and the like.

So your argument in favor of what i see as at least a mild form of type
obfusaction is that ... even more obfuscation is upcoming?

I think percpu usage should be spelled out clear and loud. We should not
pretend they are 'usual' C variables, because they are not. They are
defined in a special way, they are used via special operators. I sure
want to make sure that taking an address of one of them:

ptr = &dr7;

... looks special too.

Just look at the two 'fixes' i quoted in this discussion:

28b4e0d: x86: Rename global percpu symbol dr7 to cpu_dr7
11e6635: kernel/hw_breakpoint.c: Fix local/global shadowing

They actually 'solved' the shadowing by renaming the variables to ...
cpu_. Think about it: the 'I am percpu' prefix came right back - it's
just now present in a more volatile form and the default usage is
slightly more dangerous!

I guess i'm a bit more sensitive to percpu complications than you
because i've seen my fair share of bugs in the scheduler (and
preemptible/non-preemptible code) related to percpu code (a fair share
of it introduced by yourself ;-), so the last thing i'd like to see is
changes that are hiding its nature.

I _use_ percpu code, i dont just write the facilities ;-)

> [...] Again, I'm explaining what you should already know before
> sending email about this stuff.
> [...]
> Stupidest debate ever.

What i am making is a somewhat subtle technical argument and making any
progress on it needs at least a minimal form of a working debate. I do
not claim i am right, but still you are dismissing my arguments in a
rather nasty way.

... alas, i dont care _that_ much about this and i dont think my
concerns deserved your ad hominem attacks so i see no point in further
participating in this thread.

Thanks,

Ingo