net: Document dst->obsolete better.

Add a big comment explaining how the field works, and use defines
instead of magic constants for the values assigned to it.

Suggested by Joe Perches.

Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 3 insertions, 2 deletions

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index d1d579638092..50d2498c9284 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1221,7 +1221,7 @@ static void rt_set_nexthop(struct rtable *rt, const struct flowi4 *fl4,
 static struct rtable *rt_dst_alloc(struct net_device *dev,
                                    bool nopolicy, bool noxfrm)
 {
-        return dst_alloc(&ipv4_dst_ops, dev, 1, -1,
+        return dst_alloc(&ipv4_dst_ops, dev, 1, DST_OBSOLETE_FORCE_CHK,
                          DST_HOST | DST_NOCACHE |
                          (nopolicy ? DST_NOPOLICY : 0) |
                          (noxfrm ? DST_NOXFRM : 0));
@@ -1969,9 +1969,10 @@ static struct dst_ops ipv4_dst_blackhole_ops = {
 
 struct dst_entry *ipv4_blackhole_route(struct net *net, struct dst_entry *dst_orig)
 {
-        struct rtable *rt = dst_alloc(&ipv4_dst_blackhole_ops, NULL, 1, 0, 0);
         struct rtable *ort = (struct rtable *) dst_orig;
+        struct rtable *rt;
 
+        rt = dst_alloc(&ipv4_dst_blackhole_ops, NULL, 1, DST_OBSOLETE_NONE, 0);
         if (rt) {
                 struct dst_entry *new = &rt->dst;