bpf: Allow reuse from waiting_for_gp_ttrace list.

alloc_bulk() can reuse elements from free_by_rcu_ttrace.
Let it reuse from waiting_for_gp_ttrace as well to avoid unnecessary kmalloc().
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230706033447.54696-10-alexei.starovoitov@gmail.com

kernel/bpf/memalloc.c

@@ -212,6 +212,15 @@ static void alloc_bulk(struct bpf_mem_cache *c, int cnt, int node)
 	if (i >= cnt)
 		return;
 
+	for (; i < cnt; i++) {
+		obj = llist_del_first(&c->waiting_for_gp_ttrace);
+		if (!obj)
+			break;
+		add_obj_to_free_list(c, obj);
+	}
+	if (i >= cnt)
+		return;
+
 	memcg = get_memcg(c);
 	old_memcg = set_active_memcg(memcg);
 	for (; i < cnt; i++) {
@@ -295,12 +304,7 @@ static void do_call_rcu_ttrace(struct bpf_mem_cache *c)
 
 	WARN_ON_ONCE(!llist_empty(&c->waiting_for_gp_ttrace));
 	llist_for_each_safe(llnode, t, llist_del_all(&c->free_by_rcu_ttrace))
-		/* There is no concurrent __llist_add(waiting_for_gp_ttrace) access.
-		 * It doesn't race with llist_del_all either.
-		 * But there could be two concurrent llist_del_all(waiting_for_gp_ttrace):
-		 * from __free_rcu() and from drain_mem_cache().
-		 */
-		__llist_add(llnode, &c->waiting_for_gp_ttrace);
+		llist_add(llnode, &c->waiting_for_gp_ttrace);
 
 	if (unlikely(READ_ONCE(c->draining))) {
 		__free_rcu(&c->rcu_ttrace);