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

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Message-ID: <20091129064019.GA19916@elte.hu>
Date:	Sun, 29 Nov 2009 07:40:19 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Tejun Heo <tj@...nel.org>, 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

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

> On Fri, 27 Nov 2009 04:11:28 pm Ingo Molnar wrote:
> > 
> > * Rusty Russell <rusty@...tcorp.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 Your arguments are fine, but out-of-date.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
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