locking/mutex: Add comment to __mutex_owner() to deter usage

Attempt to deter usage, this is not a public interface. It is entirely
possible to implement a conformant mutex without having this owner
field (in fact, we used to have that).

Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Signed-off-by: Ingo Molnar <[email protected]>

Author: Peter Zijlstra

include/linux/mutex.h

@@ -66,6 +66,11 @@ struct mutex {
 #endif
 };
 
+/*
+ * Internal helper function; C doesn't allow us to hide it :/
+ *
+ * DO NOT USE (outside of mutex code).
+ */
 static inline struct task_struct *__mutex_owner(struct mutex *lock)
 {
 	return (struct task_struct *)(atomic_long_read(&lock->owner) & ~0x07);