[PATCH v5 06/36] cleanup: Basic compatibility with context analysis

[Marco Elver <elver@google.com>]

Introduce basic compatibility with cleanup.h infrastructure.

We need to allow the compiler to see the acquisition and release of the
context lock at the start and end of a scope. However, the current
"cleanup" helpers wrap the lock in a struct passed through separate
helper functions, which hides the lock alias from the compiler (no
inter-procedural analysis).

While Clang supports scoped guards in C++, it's not possible to apply in
C code: https://clang.llvm.org/docs/ThreadSafetyAnalysis.html#scoped-context

However, together with recent improvements to Clang's alias analysis
abilities, idioms such as this work correctly now:

        void spin_unlock_cleanup(spinlock_t **l) __releases(*l) { .. }
        ...
        {
            spinlock_t *lock_scope __cleanup(spin_unlock_cleanup) = &lock;
            spin_lock(&lock);  // lock through &lock
            ... critical section ...
        }  // unlock through lock_scope -[alias]-> &lock (no warnings)

To generalize this pattern and make it work with existing lock guards,
introduce DECLARE_LOCK_GUARD_1_ATTRS() and WITH_LOCK_GUARD_1_ATTRS().

These allow creating an explicit alias to the context lock instance that
is "cleaned" up with a separate cleanup helper. This helper is a dummy
function that does nothing at runtime, but has the release attributes to
tell the compiler what happens at the end of the scope.

Example usage:

  DECLARE_LOCK_GUARD_1_ATTRS(mutex, __acquires(_T), __releases(*(struct mutex **)_T))
  #define class_mutex_constructor(_T) WITH_LOCK_GUARD_1_ATTRS(mutex, _T)

Note: To support the for-loop based scoped helpers, the auxiliary
variable must be a pointer to the "class" type because it is defined in
the same statement as the guard variable. However, we initialize it with
the lock pointer (despite the type mismatch, the compiler's alias
analysis still works as expected). The "_unlock" attribute receives a
pointer to the auxiliary variable (a double pointer to the class type),
and must be cast and dereferenced appropriately.

Signed-off-by: Marco Elver <elver@google.com>
---
v5:
* Rework infrastructure to properly release at scope end with reworked
  WITH_LOCK_GUARD_1_ATTRS() and WITH_LOCK_GUARD_1_ATTRS().

v4:
* Rename capability -> context analysis.

v3:
* Add *_ATTRS helpers instead of implicit __assumes_cap (suggested by Peter)
* __assert -> __assume rename
---
 include/linux/cleanup.h | 50 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
index 8d41b917c77d..ee6df68c2177 100644
--- a/include/linux/cleanup.h
+++ b/include/linux/cleanup.h
@@ -278,16 +278,21 @@ const volatile void * __must_check_fn(const volatile void *val)
 
 #define DEFINE_CLASS(_name, _type, _exit, _init, _init_args...)		\
 typedef _type class_##_name##_t;					\
+typedef _type lock_##_name##_t;						\
 static __always_inline void class_##_name##_destructor(_type *p)	\
+	__no_context_analysis						\
 { _type _T = *p; _exit; }						\
 static __always_inline _type class_##_name##_constructor(_init_args)	\
+	__no_context_analysis						\
 { _type t = _init; return t; }
 
 #define EXTEND_CLASS(_name, ext, _init, _init_args...)			\
+typedef lock_##_name##_t lock_##_name##ext##_t;			\
 typedef class_##_name##_t class_##_name##ext##_t;			\
 static __always_inline void class_##_name##ext##_destructor(class_##_name##_t *p) \
 { class_##_name##_destructor(p); }					\
 static __always_inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
+	__no_context_analysis \
 { class_##_name##_t t = _init; return t; }
 
 #define CLASS(_name, var)						\
@@ -474,12 +479,14 @@ _label:									\
  */
 
 #define __DEFINE_UNLOCK_GUARD(_name, _type, _unlock, ...)		\
+typedef _type lock_##_name##_t;						\
 typedef struct {							\
 	_type *lock;							\
 	__VA_ARGS__;							\
 } class_##_name##_t;							\
 									\
 static __always_inline void class_##_name##_destructor(class_##_name##_t *_T) \
+	__no_context_analysis						\
 {									\
 	if (!__GUARD_IS_ERR(_T->lock)) { _unlock; }			\
 }									\
@@ -488,6 +495,7 @@ __DEFINE_GUARD_LOCK_PTR(_name, &_T->lock)
 
 #define __DEFINE_LOCK_GUARD_1(_name, _type, _lock)			\
 static __always_inline class_##_name##_t class_##_name##_constructor(_type *l) \
+	__no_context_analysis						\
 {									\
 	class_##_name##_t _t = { .lock = l }, *_T = &_t;		\
 	_lock;								\
@@ -496,6 +504,7 @@ static __always_inline class_##_name##_t class_##_name##_constructor(_type *l) \
 
 #define __DEFINE_LOCK_GUARD_0(_name, _lock)				\
 static __always_inline class_##_name##_t class_##_name##_constructor(void) \
+	__no_context_analysis						\
 {									\
 	class_##_name##_t _t = { .lock = (void*)1 },			\
 			 *_T __maybe_unused = &_t;			\
@@ -503,6 +512,47 @@ static __always_inline class_##_name##_t class_##_name##_constructor(void) \
 	return _t;							\
 }
 
+#define DECLARE_LOCK_GUARD_0_ATTRS(_name, _lock, _unlock)		\
+static inline class_##_name##_t class_##_name##_constructor(void) _lock;\
+static inline void class_##_name##_destructor(class_##_name##_t *_T) _unlock;
+
+/*
+ * To support Context Analysis, we need to allow the compiler to see the
+ * acquisition and release of the context lock. However, the "cleanup" helpers
+ * wrap the lock in a struct passed through separate helper functions, which
+ * hides the lock alias from the compiler (no inter-procedural analysis).
+ *
+ * To make it work, we introduce an explicit alias to the context lock instance
+ * that is "cleaned" up with a separate cleanup helper. This helper is a dummy
+ * function that does nothing at runtime, but has the "_unlock" attribute to
+ * tell the compiler what happens at the end of the scope.
+ *
+ * To generalize the pattern, the WITH_LOCK_GUARD_1_ATTRS() macro should be used
+ * to redefine the constructor, which then also creates the alias variable with
+ * the right "cleanup" attribute, *after* DECLARE_LOCK_GUARD_1_ATTRS() has been
+ * used.
+ *
+ * Example usage:
+ *
+ *   DECLARE_LOCK_GUARD_1_ATTRS(mutex, __acquires(_T), __releases(*(struct mutex **)_T))
+ *   #define class_mutex_constructor(_T) WITH_LOCK_GUARD_1_ATTRS(mutex, _T)
+ *
+ * Note: To support the for-loop based scoped helpers, the auxiliary variable
+ * must be a pointer to the "class" type because it is defined in the same
+ * statement as the guard variable. However, we initialize it with the lock
+ * pointer (despite the type mismatch, the compiler's alias analysis still works
+ * as expected). The "_unlock" attribute receives a pointer to the auxiliary
+ * variable (a double pointer to the class type), and must be cast and
+ * dereferenced appropriately.
+ */
+#define DECLARE_LOCK_GUARD_1_ATTRS(_name, _lock, _unlock)		\
+static inline class_##_name##_t class_##_name##_constructor(lock_##_name##_t *_T) _lock;\
+static __always_inline void __class_##_name##_cleanup_ctx(class_##_name##_t **_T) \
+	__no_context_analysis _unlock { }
+#define WITH_LOCK_GUARD_1_ATTRS(_name, _T)				\
+	class_##_name##_constructor(_T),				\
+	*__UNIQUE_ID(unlock) __cleanup(__class_##_name##_cleanup_ctx) = (void *)(unsigned long)(_T)
+
 #define DEFINE_LOCK_GUARD_1(_name, _type, _lock, _unlock, ...)		\
 __DEFINE_CLASS_IS_CONDITIONAL(_name, false);				\
 __DEFINE_UNLOCK_GUARD(_name, _type, _unlock, __VA_ARGS__)		\
-- 
2.52.0.322.g1dd061c0dc-goog