linux-kernel - Re: [PATCH 2/3] DEBUGFS: Add per cpu counters

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Date:	Tue, 13 Dec 2011 14:56:17 -0800
From:	Andi Kleen <ak@...ux.intel.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Andi Kleen <andi@...stfloor.org>, Greg KH <greg@...ah.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 2/3] DEBUGFS: Add per cpu counters

On Tue, Dec 13, 2011 at 11:43:16PM +0100, Thomas Gleixner wrote:
> On Tue, 13 Dec 2011, Andi Kleen wrote:
> >  									\
> > +	__debugfs	: AT(ADDR(__debugfs) - LOAD_OFFSET) {		\
> > +		VMLINUX_SYMBOL(__start___debugfs) = .;			\
> > +		*(__debugfs)						\
> > +		VMLINUX_SYMBOL(__stop___debugfs) = .;			\
> > +	}								\
> > +									\
> 
> ....
>   
> > +struct debugfs_counter {
> > +	unsigned __percpu *ptr;
> > +	const char *fn;
> > +	const char *name;
> > +} __attribute__((aligned(sizeof(char *))));
> > +
> > +/* Note: static doesn't work unlike DEFINE_PERCPU. Sorry. */
> > +#define DEFINE_DEBUGFS_COUNTER(name_, file)				\
> > +	DEFINE_PER_CPU(unsigned, name_ ## _counter);			\
> > +	struct debugfs_counter name_ ## _pcpu_counter __used		\
> > +	__attribute__((aligned(sizeof(char *)),section("__debugfs"),unused)) \
> > +	= { .ptr = &name_ ## _counter, .fn = file, .name = #name_ }; \
> 
> Sigh, we had that section forms an array problem more than once
> already. Why do you invent another variant and think that it will not
> explode?

I did three or four different sections like this in the past and as far
as I know none of them has exploded so far in production use.

Can you be more specific? Where exactly do you think this will
not work?

> Your alignment magic does not guarantee at all that the structs will
> form an array. The "aligned" attribute guarantees only the _MINIMUM_
> alignment for a structure, but the compiler and the linker are free to
> align on larger multiples.

> 
> See commit 654986462 for details.

Doesn't give a lot of details actually. Which target?

Note that my structure only has pointers, so there is not a lot 
of potential for "evil" alignment.

-Andi
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