linux-kernel - Re: header circular dependencies

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Date:   Tue, 4 Jan 2022 15:45:24 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        John Ogness <john.ogness@...utronix.de>
Subject: Re: header circular dependencies

On Wed 2021-12-22 21:08:39, Andy Shevchenko wrote:
> `make headerdep` is full of printk.h circular dependencies, like
> 
> include/kvm/arm_vgic.h:18: warning: recursive header inclusion
> In file included from linux/printk.h,
>                 from linux/dynamic_debug.h:188
>                 from linux/printk.h:555 <-- here

This one looks like false positive:

   + printk.h includes dynamic_debug.h when CONFIG_DYNAMIC_DEBUG_CORE
   + dynamic_debug.h includes printk.h when !CONFIG_DYNAMIC_DEBUG_CORE

But there seem to be other cycles, for example:

   + printk.h
     + dynamic_debug.h
       + jump_label.h
	 + bug.h
	   + asm/bug.h
	     + printk.h

I guess that it somehow works _only_ because printk.h includes
dynamic_debug.h late. It probably defines everything that is needed
by bug.h early enough.

> I'm wondering if it's a false positive?
> In either case, can we teach the headerdep not to complain by fixing
> the culprit?

I am scratching my head how to clean this up. All the dependencies
make sense. The main problem is that all headers provide a lot of
inlined functionality. The inlining is often important either because
of speed or because every caller needs to have its own data structure
(struct _ddebug, struct static_key).

I can't find any good solution at the moment. But I am still slowed
down after the holidays.

Best Regards,
Petr