percpu-refcount: implement percpu_ref_cancel_init()

Normally, percpu_ref_init() initializes and percpu_ref_kill()
initiates destruction which completes asynchronously.  The
asynchronous destruction can be problematic in init failure path where
the caller wants to destroy half-constructed object - distinguishing
half-constructed objects from the usual release method can be painful
for complex objects.

This patch implements percpu_ref_cancel_init() which synchronously
destroys the percpu_ref without invoking release.  To avoid
unintentional misuses, the function requires the ref to have finished
percpu_ref_init() but never used and triggers WARN otherwise.

v2: Explain the weird name and usage restriction in the function
    comment.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Kent Overstreet <koverstreet@google.com>

include/linux/percpu-refcount.h

@@ -68,6 +68,7 @@ struct percpu_ref {
 
 int __must_check percpu_ref_init(struct percpu_ref *ref,
 				 percpu_ref_func_t *release);
+void percpu_ref_cancel_init(struct percpu_ref *ref);
 void percpu_ref_kill(struct percpu_ref *ref);
 
 #define PCPU_STATUS_BITS	2

lib/percpu-refcount.c

@@ -54,6 +54,37 @@ int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release)
 	return 0;
 }
 
+/**
+ * percpu_ref_cancel_init - cancel percpu_ref_init()
+ * @ref: percpu_ref to cancel init for
+ *
+ * Once a percpu_ref is initialized, its destruction is initiated by
+ * percpu_ref_kill() and completes asynchronously, which can be painful to
+ * do when destroying a half-constructed object in init failure path.
+ *
+ * This function destroys @ref without invoking @ref->release and the
+ * memory area containing it can be freed immediately on return.  To
+ * prevent accidental misuse, it's required that @ref has finished
+ * percpu_ref_init(), whether successful or not, but never used.
+ *
+ * The weird name and usage restriction are to prevent people from using
+ * this function by mistake for normal shutdown instead of
+ * percpu_ref_kill().
+ */
+void percpu_ref_cancel_init(struct percpu_ref *ref)
+{
+	unsigned __percpu *pcpu_count = ref->pcpu_count;
+	int cpu;
+
+	WARN_ON_ONCE(atomic_read(&ref->count) != 1 + PCPU_COUNT_BIAS);
+
+	if (pcpu_count) {
+		for_each_possible_cpu(cpu)
+			WARN_ON_ONCE(*per_cpu_ptr(pcpu_count, cpu));
+		free_percpu(ref->pcpu_count);
+	}
+}
+
 static void percpu_ref_kill_rcu(struct rcu_head *rcu)
 {
 	struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);