linux-kernel - Re: [PATCH v1] cleanup: adjust scoped_guard() to avoid potential warning

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Message-ID: <Zv6RZS3bjfNcwh-B@smile.fi.intel.com>
Date: Thu, 3 Oct 2024 15:43:17 +0300
From: Andy Shevchenko <andriy.shevchenko@...el.com>
To: Przemek Kitszel <przemyslaw.kitszel@...el.com>
Cc: linux-kernel@...r.kernel.org, amadeuszx.slawinski@...ux.intel.com,
	Tony Nguyen <anthony.l.nguyen@...el.com>,
	nex.sw.ncis.osdt.itp.upstreaming@...el.com, netdev@...r.kernel.org,
	Markus Elfring <Markus.Elfring@....de>, Kees Cook <kees@...nel.org>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Dan Carpenter <dan.carpenter@...aro.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v1] cleanup: adjust scoped_guard() to avoid potential
 warning

On Thu, Oct 03, 2024 at 01:39:06PM +0200, Przemek Kitszel wrote:
> Change scoped_guard() to make reasoning about it easier for static
> analysis tools (smatch, compiler diagnostics), especially to enable them
> to tell if the given scoped_guard() is conditional (interruptible-locks,
> try-locks) or not (like simple mutex_lock()).
> 
> Add compile-time error if scoped_cond_guard() is used for non-conditional
> lock class.
> 
> Beyond easier tooling and a little shrink reported by bloat-o-meter:
> add/remove: 3/2 grow/shrink: 45/55 up/down: 1573/-2069 (-496)
> this patch enables developer to write code like:
> 
> int foo(struct my_drv *adapter)
> {
> 	scoped_guard(spinlock, &adapter->some_spinlock)
> 		return adapter->spinlock_protected_var;
> }
> 
> Current scoped_guard() implementation does not support that,
> due to compiler complaining:
> error: control reaches end of non-void function [-Werror=return-type]
> 
> Technical stuff about the change:
> scoped_guard() macro uses common idiom of using "for" statement to declare
> a scoped variable. Unfortunately, current logic is too hard for compiler
> diagnostics to be sure that there is exactly one loop step; fix that.
> 
> To make any loop so trivial that there is no above warning, it must not
> depend on any non-const variable to tell if there are more steps. There is
> no obvious solution for that in C, but one could use the compound
> statement expression with "goto" jumping past the "loop", effectively
> leaving only the subscope part of the loop semantics.
> 
> More impl details:
> one more level of macro indirection is now needed to avoid duplicating
> label names;
> I didn't spot any other place that is using the
> "for (...; goto label) if (0) label: break;" idiom, so it's not packed
> for reuse, what makes actual macros code cleaner.
> 
> There was also a need to introduce const true/false variable per lock
> class, it is used to aid compiler diagnostics reasoning about "exactly
> 1 step" loops (note that converting that to function would undo the whole
> benefit).

...

> +#define __scoped_guard_labeled(_label, _name, args...)			\
> +	for (CLASS(_name, scope)(args);					\
> +	     __guard_ptr(_name)(&scope) || !__is_cond_ptr(_name);	\
> +		     ({ goto _label; }))				\
> +		if (0)							\
> +		_label:							\
> +			break;						\
> +		else

I believe the following will folow more the style we use in the kernel:

#define __scoped_guard_labeled(_label, _name, args...)			\
	for (CLASS(_name, scope)(args);					\
	     __guard_ptr(_name)(&scope) || !__is_cond_ptr(_name);	\
		     ({ goto _label; }))				\
		if (0) {						\
_label:									\
			break;						\
		} else

...

> -	     *done = NULL; !done; done = (void *)1) \
> +	     *done = NULL; !done; done = (void *)1 +  	\

You have TABs/spaces mix in this line now.

-- 
With Best Regards,
Andy Shevchenko