Message-Id: <20241001145718.8962-1-przemyslaw.kitszel@intel.com>
Date: Tue,  1 Oct 2024 16:57:18 +0200
From: Przemek Kitszel <przemyslaw.kitszel@...el.com>
To: linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>
Cc: 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>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Dan Carpenter <dan.carpenter@...aro.org>,
	Przemek Kitszel <przemyslaw.kitszel@...el.com>,
	Andy Shevchenko <andriy.shevchenko@...el.com>
Subject: [RFC PATCH v2] Simply enable one 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]

One could argue that for such use case it would be better to use
guard(spinlock)(&adapter->some_spinlock), I disagree. I could also say
that coding with my proposed locking style is also very pleasant, as I'm
doing so for a few weeks already.

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 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
	if (0) past_the_loop:; else for (...; 1; ({goto past_the_loop}))
idiom, so it's not packed for reuse what makes actual macros code cleaner.

There was also a need to introduce const 0/1 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).

NAKed-by: Andy Shevchenko <andriy.shevchenko@...el.com>
Signed-off-by: Przemek Kitszel <przemyslaw.kitszel@...el.com>
---
Andy believes that this change is completely wrong C, the reasons
(that I disagree with of course, are in v1, below the commit message).

v2:
 remove ", 1" condition, as scoped_guard() could be used also for
 conditional locks (try-lock, irq-lock, etc) - this was pointed out by
 Dmitry Torokhov and Dan Carpenter;
 reorder macros to have them defined prior to use - Markus Elfring.

v1:
https://lore.kernel.org/netdev/20240926134347.19371-1-przemyslaw.kitszel@intel.com
---
 include/linux/cleanup.h | 23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
index a3d3e888cf1f..72dcfeb3ec13 100644
--- a/include/linux/cleanup.h
+++ b/include/linux/cleanup.h
@@ -151,12 +151,18 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
  *
  */
 
+
+#define DEFINE_CLASS_IS_CONDITIONAL(_name, _is_cond)	\
+static __maybe_unused const bool class_##_name##_is_conditional = _is_cond
+
 #define DEFINE_GUARD(_name, _type, _lock, _unlock) \
+	DEFINE_CLASS_IS_CONDITIONAL(_name, 0); \
 	DEFINE_CLASS(_name, _type, if (_T) { _unlock; }, ({ _lock; _T; }), _type _T); \
 	static inline void * class_##_name##_lock_ptr(class_##_name##_t *_T) \
 	{ return *_T; }
 
 #define DEFINE_GUARD_COND(_name, _ext, _condlock) \
+	DEFINE_CLASS_IS_CONDITIONAL(_name##_ext, 1); \
 	EXTEND_CLASS(_name, _ext, \
 		     ({ void *_t = _T; if (_T && !(_condlock)) _t = NULL; _t; }), \
 		     class_##_name##_t _T) \
@@ -167,10 +173,18 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
 	CLASS(_name, __UNIQUE_ID(guard))
 
 #define __guard_ptr(_name) class_##_name##_lock_ptr
+#define __is_cond_ptr(_name) class_##_name##_is_conditional
+
+#define scoped_guard(_name, args...)	\
+	__scoped_guard_labeled(__UNIQUE_ID(label), _name, args)
 
-#define scoped_guard(_name, args...)					\
-	for (CLASS(_name, scope)(args),					\
-	     *done = NULL; __guard_ptr(_name)(&scope) && !done; done = (void *)1)
+#define __scoped_guard_labeled(_label, _name, args...)	\
+	if (0)						\
+		_label: ;				\
+	else						\
+		for (CLASS(_name, scope)(args);		\
+		     __guard_ptr(_name)(&scope) || !__is_cond_ptr(_name); \
+		     ({goto _label;}))
 
 #define scoped_cond_guard(_name, _fail, args...) \
 	for (CLASS(_name, scope)(args), \
@@ -233,14 +247,17 @@ static inline class_##_name##_t class_##_name##_constructor(void)	\
 }
 
 #define DEFINE_LOCK_GUARD_1(_name, _type, _lock, _unlock, ...)		\
+DEFINE_CLASS_IS_CONDITIONAL(_name, 0);					\
 __DEFINE_UNLOCK_GUARD(_name, _type, _unlock, __VA_ARGS__)		\
 __DEFINE_LOCK_GUARD_1(_name, _type, _lock)
 
 #define DEFINE_LOCK_GUARD_0(_name, _lock, _unlock, ...)			\
+DEFINE_CLASS_IS_CONDITIONAL(_name, 0);					\
 __DEFINE_UNLOCK_GUARD(_name, void, _unlock, __VA_ARGS__)		\
 __DEFINE_LOCK_GUARD_0(_name, _lock)
 
 #define DEFINE_LOCK_GUARD_1_COND(_name, _ext, _condlock)		\
+	DEFINE_CLASS_IS_CONDITIONAL(_name##_ext, 1);			\
 	EXTEND_CLASS(_name, _ext,					\
 		     ({ class_##_name##_t _t = { .lock = l }, *_T = &_t;\
 		        if (_T->lock && !(_condlock)) _T->lock = NULL;	\

base-commit: c824deb1a89755f70156b5cdaf569fca80698719
-- 
2.39.3

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