percpu_ref: implement percpu_ref_is_dying()

Implement percpu_ref_is_dying() which tests whether the ref is dying
or dead.  This is useful to determine the current state when a
percpu_ref is used as a cyclic on/off switch via kill and reinit.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Kent Overstreet <kmo@daterainc.com>

include/linux/percpu-refcount.h

@@ -294,6 +294,20 @@ static inline void percpu_ref_put(struct percpu_ref *ref)
 	percpu_ref_put_many(ref, 1);
 }
 
+/**
+ * percpu_ref_is_dying - test whether a percpu refcount is dying or dead
+ * @ref: percpu_ref to test
+ *
+ * Returns %true if @ref is dying or dead.
+ *
+ * This function is safe to call as long as @ref is between init and exit
+ * and the caller is responsible for synchronizing against state changes.
+ */
+static inline bool percpu_ref_is_dying(struct percpu_ref *ref)
+{
+	return ref->percpu_count_ptr & __PERCPU_REF_DEAD;
+}
+
 /**
  * percpu_ref_is_zero - test whether a percpu refcount reached zero
  * @ref: percpu_ref to test