libc: Enforce lock-free atomic_flag and C23-safe initialisation

Select the `atomic_flag` backing type according to the C standard
requirements that `atomic_flag` operations be lock-free.

C11 §7.17.1.5 defines `atomic_flag` as:

> a structure type representing a lock-free, primitive atomic flag

and §7.17.8.2 further requires:

> Operations on an object of type atomic_flag shall be lock free

Therefore:

- Prefer `atomic_bool` when `ATOMIC_BOOL_LOCK_FREE == 2`
- Fall back to `atomic_uchar` when `ATOMIC_CHAR_LOCK_FREE == 2`
- Trigger a translation failure if neither type is lock-free

Adjust `ATOMIC_FLAG_INIT` for C23 initialisation rules:

- Use `{ ATOMIC_VAR_INIT(0) }` in pre-C23 modes
- Use `{ 0 }` in C23 and later

Preserve `atomic_flag_test_and_set_explicit()` semantics by
normalising the exchanged value with `!= 0`, ensuring consistent
boolean results regardless of whether the underlying
representation is `atomic_bool` or `atomic_uchar`.

Signed-off-by: Faraz Vahedi <kfv@kfv.io>
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/2185

1 files changed, 12 insertions, 4 deletions

diff --git a/sys/sys/stdatomic.h b/sys/sys/stdatomic.h
index bbdd196515e0..3e4bdd505418 100644
--- a/sys/sys/stdatomic.h
+++ b/sys/sys/stdatomic.h
@@ -377,21 +377,29 @@ __extension__ ({							\
 
 /*
  * 7.17.8 Atomic flag type and operations.
- *
- * XXX: Assume atomic_bool can be used as an atomic_flag. Is there some
- * kind of compiler built-in type we could use?
  */
 
 typedef struct {
+#if ATOMIC_BOOL_LOCK_FREE == 2
 	atomic_bool	__flag;
+#elif ATOMIC_CHAR_LOCK_FREE == 2
+	atomic_uchar	__flag;
+#else
+#error "atomic_flag is required to be lock-free"
+#endif
 } atomic_flag;
+#if __ISO_C_VISIBLE < 2023
 #define	ATOMIC_FLAG_INIT		{ ATOMIC_VAR_INIT(0) }
+#else
+#define	ATOMIC_FLAG_INIT		{ 0 }
+#endif
 
 static __inline _Bool
 atomic_flag_test_and_set_explicit(volatile atomic_flag *__object,
     memory_order __order)
 {
-	return (atomic_exchange_explicit(&__object->__flag, 1, __order));
+
+	return (atomic_exchange_explicit(&__object->__flag, 1, __order) != 0);
 }
 
 static __inline void