I wrote stupid runtime checker to look for atomic_t uninitialized usage
and the amount of screaming in logs is surprisingly very big.

So the question: is there really really an arch for which setting atomic_t
by hand (kzalloc) is not equivalent to atomic_set()?

Given the following patch, there is none almost certainly.

--- a/kernel/user.c
+++ b/kernel/user.c

@@ -405,6 +405,9 @@ struct user_struct *alloc_uid(struct user_namespace *ns, uid_t uid)
 
 		new->uid = uid;
 		atomic_set(&new->__count, 1);
+		atomic_set(&new->processes, 0);
+		atomic_set(&new->files, 0);
+		atomic_set(&new->sigpending, 0);
 
 		if (sched_create_user(new) < 0)
 			goto out_free_user;

Such checker will still be useful to catch genuine uninitialized usages,
but the amount of stuff to shut up before it can realistically be put in -mm
is amazing.