phonet: add __rcu annotations

Removes following sparse errors.

make C=2 net/phonet/socket.o net/phonet/af_phonet.o
CHECK net/phonet/socket.c
net/phonet/socket.c:619:14: error: incompatible types in comparison expression (different address spaces):
net/phonet/socket.c:619:14: struct sock [noderef] __rcu *
net/phonet/socket.c:619:14: struct sock *
net/phonet/socket.c:642:17: error: incompatible types in comparison expression (different address spaces):
net/phonet/socket.c:642:17: struct sock [noderef] __rcu *
net/phonet/socket.c:642:17: struct sock *
net/phonet/socket.c:658:17: error: incompatible types in comparison expression (different address spaces):
net/phonet/socket.c:658:17: struct sock [noderef] __rcu *
net/phonet/socket.c:658:17: struct sock *
net/phonet/socket.c:677:25: error: incompatible types in comparison expression (different address spaces):
net/phonet/socket.c:677:25: struct sock [noderef] __rcu *
net/phonet/socket.c:677:25: struct sock *
net/phonet/socket.c:726:21: warning: context imbalance in 'pn_res_seq_start' - wrong count at exit
net/phonet/socket.c:741:13: warning: context imbalance in 'pn_res_seq_stop' - wrong count at exit
CHECK net/phonet/af_phonet.c
net/phonet/af_phonet.c:35:14: error: incompatible types in comparison expression (different address spaces):
net/phonet/af_phonet.c:35:14: struct phonet_protocol const [noderef] __rcu *
net/phonet/af_phonet.c:35:14: struct phonet_protocol const *
net/phonet/af_phonet.c:474:17: error: incompatible types in comparison expression (different address spaces):
net/phonet/af_phonet.c:474:17: struct phonet_protocol const [noderef] __rcu *
net/phonet/af_phonet.c:474:17: struct phonet_protocol const *
net/phonet/af_phonet.c:486:9: error: incompatible types in comparison expression (different address spaces):
net/phonet/af_phonet.c:486:9: struct phonet_protocol const [noderef] __rcu *
net/phonet/af_phonet.c:486:9: struct phonet_protocol const *

Signed-off-by: Eric Dumazet <[e-mail]>
Acked-by: Rémi Denis-Courmont <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/phonet/af_phonet.c

···
 #include <net/phonet/pn_dev.h>
 
 /* Transport protocol registration */
-static const struct phonet_protocol *proto_tab[PHONET_NPROTO] __read_mostly;
+static const struct phonet_protocol __rcu *proto_tab[PHONET_NPROTO] __read_mostly;
 
 static const struct phonet_protocol *phonet_proto_get(unsigned int protocol)
 {
···
 			const struct phonet_protocol *pp)
 {
 	mutex_lock(&proto_tab_lock);
-	BUG_ON(proto_tab[protocol] != pp);
+	BUG_ON(rcu_access_pointer(proto_tab[protocol]) != pp);
 	RCU_INIT_POINTER(proto_tab[protocol], NULL);
 	mutex_unlock(&proto_tab_lock);
 	synchronize_rcu();

net/phonet/socket.c

···
 #endif
 
 static struct  {
-	struct sock *sk[256];
+	struct sock __rcu *sk[256];
 } pnres;
 
 /*
···
 		return -EPERM;
 
 	mutex_lock(&resource_mutex);
-	if (pnres.sk[res] == sk) {
+	if (rcu_access_pointer(pnres.sk[res]) == sk) {
 		RCU_INIT_POINTER(pnres.sk[res], NULL);
 		ret = 0;
 	}
···
 
 	mutex_lock(&resource_mutex);
 	for (res = 0; res < 256; res++) {
-		if (pnres.sk[res] == sk) {
+		if (rcu_access_pointer(pnres.sk[res]) == sk) {
 			RCU_INIT_POINTER(pnres.sk[res], NULL);
 			match++;
 		}
···
 }
 
 #ifdef CONFIG_PROC_FS
-static struct sock **pn_res_get_idx(struct seq_file *seq, loff_t pos)
+static struct sock __rcu **pn_res_get_idx(struct seq_file *seq, loff_t pos)
 {
 	struct net *net = seq_file_net(seq);
 	unsigned int i;
···
 		return NULL;
 
 	for (i = 0; i < 256; i++) {
-		if (pnres.sk[i] == NULL)
+		if (rcu_access_pointer(pnres.sk[i]) == NULL)
 			continue;
 		if (!pos)
 			return pnres.sk + i;
···
 	return NULL;
 }
 
-static struct sock **pn_res_get_next(struct seq_file *seq, struct sock **sk)
+static struct sock __rcu **pn_res_get_next(struct seq_file *seq, struct sock __rcu **sk)
 {
 	struct net *net = seq_file_net(seq);
 	unsigned int i;
···
 
 static void *pn_res_seq_next(struct seq_file *seq, void *v, loff_t *pos)
 {
-	struct sock **sk;
+	struct sock __rcu **sk;
 
 	if (v == SEQ_START_TOKEN)
 		sk = pn_res_get_idx(seq, 0);
···
 static int pn_res_seq_show(struct seq_file *seq, void *v)
 {
 	seq_setwidth(seq, 63);
-	if (v == SEQ_START_TOKEN)
+	if (v == SEQ_START_TOKEN) {
 		seq_puts(seq, "rs   uid inode");
-	else {
-		struct sock **psk = v;
-		struct sock *sk = *psk;
+	} else {
+		struct sock __rcu **psk = v;
+		struct sock *sk = rcu_dereference_protected(*psk,
+					lockdep_is_held(&resource_mutex));
 
 		seq_printf(seq, "%02X %5u %lu",
 			   (int) (psk - pnres.sk),