neighbour: Protect tbl->phash_buckets[] with a dedicated mutex.

tbl->phash_buckets[] is only modified in the slow path by pneigh_create()
and pneigh_delete() under the table lock.

Both of them are called under RTNL, so no extra lock is needed, but we
will remove RTNL from the paths.

pneigh_create() looks up a pneigh_entry, and this part can be lockless,
but it would complicate the logic like

  1. lookup
  2. allocate pengih_entry for GFP_KERNEL
  3. lookup again but under lock
  4. if found, return it after freeing the allocated memory
  5. else, return the new one

Instead, let's add a per-table mutex and run lookup and allocation
under it.

Note that updating pneigh_entry part in neigh_add() is still protected
by RTNL and will be moved to pneigh_create() in the next patch.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250716221221.442239-15-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

2 files changed, 22 insertions, 18 deletions

diff --git a/include/net/neighbour.h b/include/net/neighbour.h
index f8c7261cd4eb..f333f9ebc425 100644
--- a/include/net/neighbour.h
+++ b/include/net/neighbour.h
@@ -240,6 +240,7 @@ struct neigh_table {
 	unsigned long		last_rand;
 	struct neigh_statistics	__percpu *stats;
 	struct neigh_hash_table __rcu *nht;
+	struct mutex		phash_lock;
 	struct pneigh_entry	__rcu **phash_buckets;
 };
 
diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 38f0067068c5..d312b6323ff2 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -54,9 +54,8 @@ static void neigh_timer_handler(struct timer_list *t);
 static void __neigh_notify(struct neighbour *n, int type, int flags,
 			   u32 pid);
 static void neigh_update_notify(struct neighbour *neigh, u32 nlmsg_pid);
-static void pneigh_ifdown_and_unlock(struct neigh_table *tbl,
-				     struct net_device *dev,
-				     bool skip_perm);
+static void pneigh_ifdown(struct neigh_table *tbl, struct net_device *dev,
+			  bool skip_perm);
 
 #ifdef CONFIG_PROC_FS
 static const struct seq_operations neigh_stat_seq_ops;
@@ -437,7 +436,9 @@ static int __neigh_ifdown(struct neigh_table *tbl, struct net_device *dev,
 {
 	write_lock_bh(&tbl->lock);
 	neigh_flush_dev(tbl, dev, skip_perm);
-	pneigh_ifdown_and_unlock(tbl, dev, skip_perm);
+	write_unlock_bh(&tbl->lock);
+
+	pneigh_ifdown(tbl, dev, skip_perm);
 	pneigh_queue_purge(&tbl->proxy_queue, dev ? dev_net(dev) : NULL,
 			   tbl->family);
 	if (skb_queue_empty_lockless(&tbl->proxy_queue))
@@ -731,7 +732,7 @@ struct pneigh_entry *pneigh_lookup(struct neigh_table *tbl,
 	key_len = tbl->key_len;
 	hash_val = pneigh_hash(pkey, key_len);
 	n = rcu_dereference_check(tbl->phash_buckets[hash_val],
-				  lockdep_is_held(&tbl->lock));
+				  lockdep_is_held(&tbl->phash_lock));
 
 	while (n) {
 		if (!memcmp(n->key, pkey, key_len) &&
@@ -739,7 +740,7 @@ struct pneigh_entry *pneigh_lookup(struct neigh_table *tbl,
 		    (n->dev == dev || !n->dev))
 			return n;
 
-		n = rcu_dereference_check(n->next, lockdep_is_held(&tbl->lock));
+		n = rcu_dereference_check(n->next, lockdep_is_held(&tbl->phash_lock));
 	}
 
 	return NULL;
@@ -754,11 +755,9 @@ struct pneigh_entry *pneigh_create(struct neigh_table *tbl,
 	unsigned int key_len;
 	u32 hash_val;
 
-	ASSERT_RTNL();
+	mutex_lock(&tbl->phash_lock);
 
-	read_lock_bh(&tbl->lock);
 	n = pneigh_lookup(tbl, net, pkey, dev);
-	read_unlock_bh(&tbl->lock);
 	if (n)
 		goto out;
 
@@ -780,11 +779,10 @@ struct pneigh_entry *pneigh_create(struct neigh_table *tbl,
 	}
 
 	hash_val = pneigh_hash(pkey, key_len);
-	write_lock_bh(&tbl->lock);
 	n->next = tbl->phash_buckets[hash_val];
 	rcu_assign_pointer(tbl->phash_buckets[hash_val], n);
-	write_unlock_bh(&tbl->lock);
 out:
+	mutex_unlock(&tbl->phash_lock);
 	return n;
 }
 
@@ -806,14 +804,16 @@ int pneigh_delete(struct neigh_table *tbl, struct net *net, const void *pkey,
 	key_len = tbl->key_len;
 	hash_val = pneigh_hash(pkey, key_len);
 
-	write_lock_bh(&tbl->lock);
+	mutex_lock(&tbl->phash_lock);
+
 	for (np = &tbl->phash_buckets[hash_val];
 	     (n = rcu_dereference_protected(*np, 1)) != NULL;
 	     np = &n->next) {
 		if (!memcmp(n->key, pkey, key_len) && n->dev == dev &&
 		    net_eq(pneigh_net(n), net)) {
 			rcu_assign_pointer(*np, n->next);
-			write_unlock_bh(&tbl->lock);
+
+			mutex_unlock(&tbl->phash_lock);
 
 			if (tbl->pdestructor)
 				tbl->pdestructor(n);
@@ -822,18 +822,20 @@ int pneigh_delete(struct neigh_table *tbl, struct net *net, const void *pkey,
 			return 0;
 		}
 	}
-	write_unlock_bh(&tbl->lock);
+
+	mutex_unlock(&tbl->phash_lock);
 	return -ENOENT;
 }
 
-static void pneigh_ifdown_and_unlock(struct neigh_table *tbl,
-				     struct net_device *dev,
-				     bool skip_perm)
+static void pneigh_ifdown(struct neigh_table *tbl, struct net_device *dev,
+			  bool skip_perm)
 {
 	struct pneigh_entry *n, __rcu **np;
 	LIST_HEAD(head);
 	u32 h;
 
+	mutex_lock(&tbl->phash_lock);
+
 	for (h = 0; h <= PNEIGH_HASHMASK; h++) {
 		np = &tbl->phash_buckets[h];
 		while ((n = rcu_dereference_protected(*np, 1)) != NULL) {
@@ -849,7 +851,7 @@ skip:
 		}
 	}
 
-	write_unlock_bh(&tbl->lock);
+	mutex_unlock(&tbl->phash_lock);
 
 	while (!list_empty(&head)) {
 		n = list_first_entry(&head, typeof(*n), free_node);
@@ -1796,6 +1798,7 @@ void neigh_table_init(int index, struct neigh_table *tbl)
 		WARN_ON(tbl->entry_size % NEIGH_PRIV_ALIGN);
 
 	rwlock_init(&tbl->lock);
+	mutex_init(&tbl->phash_lock);
 
 	INIT_DEFERRABLE_WORK(&tbl->gc_work, neigh_periodic_work);
 	queue_delayed_work(system_power_efficient_wq, &tbl->gc_work,