inetpeer: do not use zero refcnt for freed entries

Followup of commit aa1039e73cc2 (inetpeer: RCU conversion)

Unused inet_peer entries have a null refcnt.

Using atomic_inc_not_zero() in rcu lookups is not going to work for
them, and slow path is taken.

Fix this using -1 marker instead of 0 for deleted entries.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 8 insertions, 2 deletions

diff --git a/net/ipv4/inetpeer.c b/net/ipv4/inetpeer.c
index 58fbc7e2475e..349249fad2db 100644
--- a/net/ipv4/inetpeer.c
+++ b/net/ipv4/inetpeer.c
@@ -187,7 +187,12 @@ static struct inet_peer *lookup_rcu_bh(__be32 daddr)
 
         while (u != peer_avl_empty) {
                 if (daddr == u->v4daddr) {
-                        if (unlikely(!atomic_inc_not_zero(&u->refcnt)))
+                        /* Before taking a reference, check if this entry was
+                         * deleted, unlink_from_pool() sets refcnt=-1 to make
+                         * distinction between an unused entry (refcnt=0) and
+                         * a freed one.
+                         */
+                        if (unlikely(!atomic_add_unless(&u->refcnt, 1, -1)))
                                 u = NULL;
                         return u;
                 }
@@ -322,8 +327,9 @@ static void unlink_from_pool(struct inet_peer *p)
          * in cleanup() function to prevent sudden disappearing.  If we can
          * atomically (because of lockless readers) take this last reference,
          * it's safe to remove the node and free it later.
+         * We use refcnt=-1 to alert lockless readers this entry is deleted.
          */
-        if (atomic_cmpxchg(&p->refcnt, 1, 0) == 1) {
+        if (atomic_cmpxchg(&p->refcnt, 1, -1) == 1) {
                 struct inet_peer **stack[PEER_MAXDEPTH];
                 struct inet_peer ***stackptr, ***delp;
                 if (lookup(p->v4daddr, stack) != p)