[PATCH v5 3/8] cleanup.h: Introduce DEFINE_INACTIVE_GUARD and activate_guard

[Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>]

In scenarios where the scope of the guard differs from the scope of its
activation, introduce DEFINE_INACTIVE_GUARD() and activate_guard().

Here is an example use for a conditionally activated guard variable:

void func(bool a)
{
	DEFINE_INACTIVE_GUARD(preempt_notrace, myguard);

	[...]
	if (a) {
		might_sleep();
		activate_guard(preempt_notrace, myguard)();
	}
	[ protected code ]
}

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 include/linux/cleanup.h | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
index 04f03ad5f25d..d6a3d8099d77 100644
--- a/include/linux/cleanup.h
+++ b/include/linux/cleanup.h
@@ -146,12 +146,20 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
  *      similar to scoped_guard(), except it does fail when the lock
  *      acquire fails.
  *
+ * DEFINE_INACTIVE_GUARD(name, var):
+ *      define an inactive guard variable in a given scope, initialized to NULL.
+ *
+ * activate_guard(name, var)(args...):
+ *      activate a guard variable with its constructor, if it is not already
+ *      activated.
  */
 
 #define DECLARE_GUARD(_name, _type, _lock, _unlock) \
 	DECLARE_CLASS(_name, _type, if (_T) { _unlock; }, ({ _lock; _T; }), _type _T); \
 	static inline void * class_##_name##_lock_ptr(class_##_name##_t *_T) \
-	{ return *_T; }
+	{ return *_T; } \
+	static inline class_##_name##_t class_##_name##_null(void) \
+	{ return NULL; }
 
 #define DECLARE_GUARD_COND(_name, _ext, _condlock) \
 	EXTEND_CLASS(_name, _ext, \
@@ -175,6 +183,14 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
 		if (!__guard_ptr(_name)(&scope)) _fail; \
 		else
 
+#define DEFINE_INACTIVE_GUARD(_name, _var) \
+	class_##_name##_t _var __cleanup(class_##_name##_destructor) = \
+		class_##_name##_null()
+
+#define activate_guard(_name, _var) \
+	if (!class_##_name##_lock_ptr(&(_var))) \
+		_var = class_##_name##_constructor
+
 /*
  * Additional helper macros for generating lock guards with types, either for
  * locks that don't have a native type (eg. RCU, preempt) or those that need a
@@ -209,6 +225,11 @@ static inline void class_##_name##_destructor(class_##_name##_t *_T)	\
 static inline void *class_##_name##_lock_ptr(class_##_name##_t *_T)	\
 {									\
 	return _T->lock;						\
+}									\
+static inline class_##_name##_t class_##_name##_null(void)		\
+{									\
+	class_##_name##_t _t = { .lock = NULL };			\
+	return _t;							\
 }
 
 
-- 
2.39.2