rtnetlink: change nlk->cb_mutex role

In commit af65bdfc ("[NETLINK]: Switch cb_lock spinlock
to mutex and allow to override it"), Patrick McHardy used
a common mutex to protect both nlk->cb and the dump() operations.

The override is used for rtnl dumps, registered with
rntl_register() and rntl_register_module().

We want to be able to opt-out some dump() operations
to not acquire RTNL, so we need to protect nlk->cb
with a per socket mutex.

This patch renames nlk->cb_def_mutex to nlk->nl_cb_mutex

The optional pointer to the mutex used to protect dump()
call is stored in nlk->dump_cb_mutex
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netlink/af_netlink.c

@@ -636,7 +636,7 @@ static struct proto netlink_proto = {
 };
 
 static int __netlink_create(struct net *net, struct socket *sock,
-			    struct mutex *cb_mutex, int protocol,
+			    struct mutex *dump_cb_mutex, int protocol,
 			    int kern)
 {
 	struct sock *sk;
@@ -651,15 +651,11 @@ static int __netlink_create(struct net *net, struct socket *sock,
 	sock_init_data(sock, sk);
 
 	nlk = nlk_sk(sk);
-	if (cb_mutex) {
-		nlk->cb_mutex = cb_mutex;
-	} else {
-		nlk->cb_mutex = &nlk->cb_def_mutex;
-		mutex_init(nlk->cb_mutex);
-		lockdep_set_class_and_name(nlk->cb_mutex,
+	mutex_init(&nlk->nl_cb_mutex);
+	lockdep_set_class_and_name(&nlk->nl_cb_mutex,
 					   nlk_cb_mutex_keys + protocol,
 					   nlk_cb_mutex_key_strings[protocol]);
-	}
+	nlk->dump_cb_mutex = dump_cb_mutex;
 	init_waitqueue_head(&nlk->wait);
 
 	sk->sk_destruct = netlink_sock_destruct;
@@ -2209,7 +2205,7 @@ static int netlink_dump(struct sock *sk, bool lock_taken)
 	int alloc_size;
 
 	if (!lock_taken)
-		mutex_lock(nlk->cb_mutex);
+		mutex_lock(&nlk->nl_cb_mutex);
 	if (!nlk->cb_running) {
 		err = -EINVAL;
 		goto errout_skb;
@@ -2261,14 +2257,22 @@ static int netlink_dump(struct sock *sk, bool lock_taken)
 	netlink_skb_set_owner_r(skb, sk);
 
 	if (nlk->dump_done_errno > 0) {
+		struct mutex *extra_mutex = nlk->dump_cb_mutex;
+
 		cb->extack = &extack;
+
+		if (extra_mutex)
+			mutex_lock(extra_mutex);
 		nlk->dump_done_errno = cb->dump(skb, cb);
+		if (extra_mutex)
+			mutex_unlock(extra_mutex);
+
 		cb->extack = NULL;
 	}
 
 	if (nlk->dump_done_errno > 0 ||
 	    skb_tailroom(skb) < nlmsg_total_size(sizeof(nlk->dump_done_errno))) {
-		mutex_unlock(nlk->cb_mutex);
+		mutex_unlock(&nlk->nl_cb_mutex);
 
 		if (sk_filter(sk, skb))
 			kfree_skb(skb);
@@ -2302,13 +2306,13 @@ static int netlink_dump(struct sock *sk, bool lock_taken)
 	WRITE_ONCE(nlk->cb_running, false);
 	module = cb->module;
 	skb = cb->skb;
-	mutex_unlock(nlk->cb_mutex);
+	mutex_unlock(&nlk->nl_cb_mutex);
 	module_put(module);
 	consume_skb(skb);
 	return 0;
 
 errout_skb:
-	mutex_unlock(nlk->cb_mutex);
+	mutex_unlock(&nlk->nl_cb_mutex);
 	kfree_skb(skb);
 	return err;
 }
@@ -2331,7 +2335,7 @@ int __netlink_dump_start(struct sock *ssk, struct sk_buff *skb,
 	}
 
 	nlk = nlk_sk(sk);
-	mutex_lock(nlk->cb_mutex);
+	mutex_lock(&nlk->nl_cb_mutex);
 	/* A dump is in progress... */
 	if (nlk->cb_running) {
 		ret = -EBUSY;
@@ -2382,7 +2386,7 @@ int __netlink_dump_start(struct sock *ssk, struct sk_buff *skb,
 	module_put(control->module);
 error_unlock:
 	sock_put(sk);
-	mutex_unlock(nlk->cb_mutex);
+	mutex_unlock(&nlk->nl_cb_mutex);
 error_free:
 	kfree_skb(skb);
 	return ret;

net/netlink/af_netlink.h

@@ -39,8 +39,9 @@ struct netlink_sock {
 	bool			cb_running;
 	int			dump_done_errno;
 	struct netlink_callback	cb;
-	struct mutex		*cb_mutex;
-	struct mutex		cb_def_mutex;
+	struct mutex		nl_cb_mutex;
+
+	struct mutex		*dump_cb_mutex;
 	void			(*netlink_rcv)(struct sk_buff *skb);
 	int			(*netlink_bind)(struct net *net, int group);
 	void			(*netlink_unbind)(struct net *net, int group);