jump_label: Add RELEASE barrier after text changes

In the unlikely case text modification does not fully order things,
add some extra ordering of our own to ensure we only enabled the fast
path after all text is visible.
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Jason Baron <jbaron@akamai.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: NIngo Molnar <mingo@kernel.org>

kernel/jump_label.c

@@ -108,7 +108,11 @@ void static_key_slow_inc(struct static_key *key)
 	if (atomic_read(&key->enabled) == 0) {
 		atomic_set(&key->enabled, -1);
 		jump_label_update(key);
-		atomic_set(&key->enabled, 1);
+		/*
+		 * Ensure that if the above cmpxchg loop observes our positive
+		 * value, it must also observe all the text changes.
+		 */
+		atomic_set_release(&key->enabled, 1);
 	} else {
 		atomic_inc(&key->enabled);
 	}
@@ -130,7 +134,10 @@ void static_key_enable(struct static_key *key)
 	if (atomic_read(&key->enabled) == 0) {
 		atomic_set(&key->enabled, -1);
 		jump_label_update(key);
-		atomic_set(&key->enabled, 1);
+		/*
+		 * See static_key_slow_inc().
+		 */
+		atomic_set_release(&key->enabled, 1);
 	}
 	jump_label_unlock();
 	cpus_read_unlock();