linux-kernel - Re: [RFC 1/2] printk/dynamic_debug: Remove cyclic dependency between printk.h and dynamic

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Date:   Wed, 12 Jan 2022 13:12:27 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        John Ogness <john.ogness@...utronix.de>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Chris Down <chris@...isdown.name>,
        Marc Zyngier <maz@...nel.org>,
        Andrew Scull <ascull@...gle.com>,
        Will Deacon <will@...nel.org>, Jason Baron <jbaron@...mai.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        linux-kernel@...r.kernel.org,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Jessica Yu <jeyu@...nel.org>, Jim Cromie <jim.cromie@...il.com>
Subject: Re: [RFC 1/2] printk/dynamic_debug: Remove cyclic dependency between
 printk.h and dynamic_debug.h

On Tue 2022-01-11 17:01:35, Rasmus Villemoes wrote:
> On 11/01/2022 15.30, Petr Mladek wrote:
> > `make headerdep` reports many circular dependencies with the same
> > pattern:
> > 
> > In file included from linux/printk.h,
> >                  from linux/dynamic_debug.h:188
> >                  from linux/printk.h:559 <-- here
> > 
> > It 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
> > 
> > Instead, this patch adds 'include/linux/printk_core.h' and moves some
> > lowlevel printk API there. Then the raw _printk() can be called from
> > the inlined code in 'dynamic_debug.h'.
> 
> Urgh, this doesn't look like the right approach.

It is not ideal. But it allows to handle these cycles without
complicating external code. It is not only about dynamic_debug.h.
The problem is also in include/asm-generic/bug.h and possibly other
headers included directly or indirectly from printk.h.

Another small advantage is that printk_core.h might define other
printk API that is not intended for general use.


> >  static inline int ddebug_add_module(struct _ddebug *tab, unsigned int n,
> >  				    const char *modname)
> > @@ -202,9 +202,8 @@ static inline int ddebug_dyndbg_module_param_cb(char *param, char *val,
> >  						const char *modname)
> >  {
> >  	if (strstr(param, "dyndbg")) {
> > -		/* avoid pr_warn(), which wants pr_fmt() fully defined */
> > -		printk(KERN_WARNING "dyndbg param is supported only in "
> > -			"CONFIG_DYNAMIC_DEBUG builds\n");
> > +		/* Use raw _printk() to avoid cyclic dependency. */
> > +		_printk(KERN_WARNING "dyndbg param is supported only in CONFIG_DYNAMIC_DEBUG builds\n");
> >  		return 0; /* allow and ignore */
> >  	}
> >  	return -EINVAL;
> 
> It looks like this has only one caller, kernel/module.c. I suggest
> simply moving the match logic into unknown_module_param_cb(), making it
> on par with the other "generic" module parameter async_probe. That is,
> do something like
> 
>   if (strstr(param, "dyndbg")) {
>     if (IS_ENABLED(CONFIG_DYNAMIC_DEBUG)) {
>       return ddebug_dyndbg_module_param_cb(param, val, modname)
>     }
>     pr_warn("dyndbg param is supported only in ...");
>     return 0; /* allow and ignore */
>   }
> 
>   pr_warn("%s: unknown parameter '%s' ignored\n", modname, param);
>   return 0;
> 
> That makes it simpler to add more magic/generic module parameters in
> unknown_module_param_cb(). No need for a static inline stub, and no need
> for conditionally declaring ddebug_dyndbg_module_param_cb(). So all that
> is needed in dynamic_debug.h is to remove the static inline definition,
> and pull the declaration outside #if defined(CONFIG_DYNAMIC_DEBUG_CORE)
> protection.

I do not have strong opinion here. The advantage of the current code
is that it keeps the complexity in dynamic_debug code.

Adding Jessica into CC to know her preferences.


> What's with the strstr, btw? Shouldn't it just be !strcmp()?

"dyndbg" parameter might be used as <module>.dyndbg[="val"].


> > @@ -223,7 +222,8 @@ static inline int ddebug_dyndbg_module_param_cb(char *param, char *val,
> >  
> >  static inline int dynamic_debug_exec_queries(const char *query, const char *modname)
> >  {
> > -	pr_warn("kernel not built with CONFIG_DYNAMIC_DEBUG_CORE\n");
> > +	/* Use raw _printk() to avoid cyclic dependency. */
> > +	_printk(KERN_WARNING "kernel not built with CONFIG_DYNAMIC_DEBUG_CORE\n");
> >  	return 0;
> >  }
> 
> And for this one I think the solution is even simpler, as I can't find
> any in-tree callers. Perhaps just nuke it entirely?

Adding Jim into Cc whether he still has any plans to use this API.

Best Regards,
Petr