Like ppp_generic.c, convert the PPPoE socket hash table to use RCU for
lookups and a spinlock for updates. This removes rwlock usage and allows
lockless readers on the fast path.

- Mark hash table and list pointers as __rcu.
- Use spin_lock() to protect writers.
- Readers use rcu_dereference() under rcu_read_lock(). All known callers
  of get_item() already hold the RCU read lock, so no additional locking
  is needed.
- Set SOCK_RCU_FREE to defer socket freeing until after an RCU grace
  period.

Signed-off-by: Qingfang Deng <dqfext@gmail.com>
---
 drivers/net/ppp/pppoe.c  | 83 ++++++++++++++++++++++------------------
 include/linux/if_pppox.h |  2 +-
 2 files changed, 46 insertions(+), 39 deletions(-)

diff --git a/drivers/net/ppp/pppoe.c b/drivers/net/ppp/pppoe.c
index 410effa42ade..f99533c80b66 100644
--- a/drivers/net/ppp/pppoe.c
+++ b/drivers/net/ppp/pppoe.c

@@ -100,8 +100,8 @@ struct pppoe_net {
         * as well, moreover in case of SMP less locking
         * controversy here
         */
-       struct pppox_sock *hash_table[PPPOE_HASH_SIZE];
-       rwlock_t hash_lock;
+       struct pppox_sock __rcu *hash_table[PPPOE_HASH_SIZE];
+       spinlock_t hash_lock;
 };

 /*

@@ -162,13 +162,13 @@ static struct pppox_sock *__get_item(struct pppoe_net *pn, __be16 sid,
        int hash = hash_item(sid, addr);
        struct pppox_sock *ret;

-       ret = pn->hash_table[hash];
+       ret = rcu_dereference(pn->hash_table[hash]);
        while (ret) {
                if (cmp_addr(&ret->pppoe_pa, sid, addr) &&
                    ret->pppoe_ifindex == ifindex)
                        return ret;

-               ret = ret->next;
+               ret = rcu_dereference(ret->next);
        }

        return NULL;

@@ -177,19 +177,20 @@ static struct pppox_sock *__get_item(struct pppoe_net *pn, __be16 sid,
 static int __set_item(struct pppoe_net *pn, struct pppox_sock *po)
 {
        int hash = hash_item(po->pppoe_pa.sid, po->pppoe_pa.remote);
-       struct pppox_sock *ret;
+       struct pppox_sock *ret, *first;

-       ret = pn->hash_table[hash];
+       first = rcu_dereference_protected(pn->hash_table[hash], lockdep_is_held(&pn->hash_lock));
+       ret = first;
        while (ret) {
                if (cmp_2_addr(&ret->pppoe_pa, &po->pppoe_pa) &&
                    ret->pppoe_ifindex == po->pppoe_ifindex)
                        return -EALREADY;

-               ret = ret->next;
+               ret = rcu_dereference_protected(ret->next, lockdep_is_held(&pn->hash_lock));
        }

-       po->next = pn->hash_table[hash];
-       pn->hash_table[hash] = po;
+       RCU_INIT_POINTER(po->next, first);
+       rcu_assign_pointer(pn->hash_table[hash], po);

        return 0;
 }

@@ -198,20 +199,24 @@ static void __delete_item(struct pppoe_net *pn, __be16 sid,
                                        char *addr, int ifindex)
 {
        int hash = hash_item(sid, addr);
-       struct pppox_sock *ret, **src;
+       struct pppox_sock *ret, __rcu **src;

-       ret = pn->hash_table[hash];
+       ret = rcu_dereference_protected(pn->hash_table[hash], lockdep_is_held(&pn->hash_lock));
        src = &pn->hash_table[hash];

        while (ret) {
                if (cmp_addr(&ret->pppoe_pa, sid, addr) &&
                    ret->pppoe_ifindex == ifindex) {
-                       *src = ret->next;
+                       struct pppox_sock *next;
+
+                       next = rcu_dereference_protected(ret->next,
+                                                        lockdep_is_held(&pn->hash_lock));
+                       rcu_assign_pointer(*src, next);
                        break;
                }

                src = &ret->next;
-               ret = ret->next;
+               ret = rcu_dereference_protected(ret->next, lockdep_is_held(&pn->hash_lock));
        }
 }

@@ -225,11 +230,9 @@ static inline struct pppox_sock *get_item(struct pppoe_net *pn, __be16 sid,
 {
        struct pppox_sock *po;

-       read_lock_bh(&pn->hash_lock);
        po = __get_item(pn, sid, addr, ifindex);
        if (po)
                sock_hold(sk_pppox(po));