net: annotate data-races around cleanup_net_task

from_cleanup_net() reads cleanup_net_task locklessly.

Add READ_ONCE()/WRITE_ONCE() annotations to avoid
a potential KCSAN warning, even if the race is harmless.

Fixes: 0734d7c ("net: expedite synchronize_net() for cleanup_net()")
Signed-off-by: Eric Dumazet <[email protected]>
Reviewed-by: Jason Xing <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: edumazet

net/core/dev.c

@@ -10499,7 +10499,7 @@ static void dev_index_release(struct net *net, int ifindex)
 static bool from_cleanup_net(void)
 {
 #ifdef CONFIG_NET_NS
-	return current == cleanup_net_task;
+	return current == READ_ONCE(cleanup_net_task);
 #else
 	return false;
 #endif

net/core/net_namespace.c

@@ -654,7 +654,7 @@ static void cleanup_net(struct work_struct *work)
 	struct net *net, *tmp, *last;
 	LIST_HEAD(net_exit_list);
 
-	cleanup_net_task = current;
+	WRITE_ONCE(cleanup_net_task, current);
 
 	/* Atomically snapshot the list of namespaces to cleanup */
 	net_kill_list = llist_del_all(&cleanup_list);
@@ -704,7 +704,7 @@ static void cleanup_net(struct work_struct *work)
 		put_user_ns(net->user_ns);
 		net_passive_dec(net);
 	}
-	cleanup_net_task = NULL;
+	WRITE_ONCE(cleanup_net_task, NULL);
 }
 
 /**