xfrm: policy: use atomic_inc_not_zero in rcu section

If we don't hold the policy lock anymore the refcnt might
already be 0, i.e. policy struct is about to be free'd.

Switch to atomic_inc_not_zero to avoid this.

On removal policies are already unlinked from the tables (lists)
before the last _put occurs so we are not supposed to find the same
'dead' entry on the next loop, so its safe to just repeat the lookup.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_policy.c

@@ -60,6 +60,11 @@ static void __xfrm_policy_link(struct xfrm_policy *pol, int dir);
 static struct xfrm_policy *__xfrm_policy_unlink(struct xfrm_policy *pol,
 						int dir);
 
+static inline bool xfrm_pol_hold_rcu(struct xfrm_policy *policy)
+{
+	return atomic_inc_not_zero(&policy->refcnt);
+}
+
 static inline bool
 __xfrm4_selector_match(const struct xfrm_selector *sel, const struct flowi *fl)
 {
@@ -1164,7 +1169,8 @@ static struct xfrm_policy *xfrm_policy_lookup_bytype(struct net *net, u8 type,
 	if (read_seqcount_retry(&xfrm_policy_hash_generation, sequence))
 		goto retry;
 
-	xfrm_pol_hold(ret);
+	if (ret && !xfrm_pol_hold_rcu(ret))
+		goto retry;
 fail:
 	read_unlock_bh(&net->xfrm.xfrm_policy_lock);