jump_labels: API for flushing deferred jump label updates

commit b6416e61 upstream.

Modules that use static_key_deferred need a way to synchronize with
any delayed work that is still pending when the module is unloaded.
Introduce static_key_deferred_flush() which flushes any pending
jump label updates.

[js] no STATIC_KEY_CHECK_USE in 3.12 -> remove it
Signed-off-by: David Matlack <dmatlack@google.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>

include/linux/jump_label_ratelimit.h

@@ -14,6 +14,7 @@ struct static_key_deferred {
 
 #ifdef HAVE_JUMP_LABEL
 extern void static_key_slow_dec_deferred(struct static_key_deferred *key);
+extern void static_key_deferred_flush(struct static_key_deferred *key);
 extern void
 jump_label_rate_limit(struct static_key_deferred *key, unsigned long rl);
 
@@ -25,6 +26,9 @@ static inline void static_key_slow_dec_deferred(struct static_key_deferred *key)
 {
 	static_key_slow_dec(&key->key);
 }
+static inline void static_key_deferred_flush(struct static_key_deferred *key)
+{
+}
 static inline void
 jump_label_rate_limit(struct static_key_deferred *key,
 		unsigned long rl)

kernel/jump_label.c

@@ -113,6 +113,12 @@ void static_key_slow_dec_deferred(struct static_key_deferred *key)
 }
 EXPORT_SYMBOL_GPL(static_key_slow_dec_deferred);
 
+void static_key_deferred_flush(struct static_key_deferred *key)
+{
+	flush_delayed_work(&key->work);
+}
+EXPORT_SYMBOL_GPL(static_key_deferred_flush);
+
 void jump_label_rate_limit(struct static_key_deferred *key,
 		unsigned long rl)
 {