[NEIGH] Fix add_timer race in neigh_add_timer

neigh_add_timer cannot use add_timer unconditionally.  The reason is that
by the time it has obtained the write lock someone else (e.g., neigh_update)
could have already added a new timer.

So it should only use mod_timer and deal with its return value accordingly.

This bug would have led to rare neighbour cache entry leaks.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

1 files changed, 2 insertions, 2 deletions

diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 766caa0dd930..37d8d8c29522 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -816,10 +816,10 @@ static void neigh_timer_handler(unsigned long arg)
         }
 
         if (neigh->nud_state & NUD_IN_TIMER) {
-                neigh_hold(neigh);
                 if (time_before(next, jiffies + HZ/2))
                         next = jiffies + HZ/2;
-                neigh_add_timer(neigh, next);
+                if (!mod_timer(&neigh->timer, next))
+                        neigh_hold(neigh);
         }
         if (neigh->nud_state & (NUD_INCOMPLETE | NUD_PROBE)) {
                 struct sk_buff *skb = skb_peek(&neigh->arp_queue);