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

Browse source 
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>
This commit is contained in:
Kuniyuki Iwashima 2025-07-16 22:08:19 +00:00 • committed by Jakub Kicinski
parent b8b7ed1ea8
commit 13a936bb99
2 changed files with 22 additions and 18 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

1
include/net/neighbour.h
View file

@ -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;
};

35
net/core/neighbour.c
View file

@ -54,8 +54,7 @@ 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,
static void pneigh_ifdown(struct neigh_table *tbl, struct net_device *dev,
bool skip_perm);
#ifdef CONFIG_PROC_FS
@ -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,
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,