Revert "ipv6: grab rt->rt6i_ref before allocating pcpu rt"

This reverts commit 9704f8147e which was
upstream commit a94b9367e0.

Shouldn't have been here, sorry about that.

Reported-by: Chris Rankin <rankincj@gmail.com>
Reported-by: Willy Tarreau <w@1wt.eu>
Cc: Ido Schimmel <idosch@idosch.org>
Cc: Ozgur <ozgur@goosey.org>
Cc: Wei Wang <weiwan@google.com>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

net/ipv6/route.c

@@ -1055,6 +1055,7 @@ static struct rt6_info *rt6_get_pcpu_route(struct rt6_info *rt)
 
 static struct rt6_info *rt6_make_pcpu_route(struct rt6_info *rt)
 {
+	struct fib6_table *table = rt->rt6i_table;
 	struct rt6_info *pcpu_rt, *prev, **p;
 
 	pcpu_rt = ip6_rt_pcpu_alloc(rt);
@@ -1065,20 +1066,28 @@ static struct rt6_info *rt6_make_pcpu_route(struct rt6_info *rt)
 		return net->ipv6.ip6_null_entry;
 	}
 
-	dst_hold(&pcpu_rt->dst);
-	p = this_cpu_ptr(rt->rt6i_pcpu);
-	prev = cmpxchg(p, NULL, pcpu_rt);
-	if (prev) {
-		/* If someone did it before us, return prev instead */
-		/* release refcnt taken by ip6_rt_pcpu_alloc() */
+	read_lock_bh(&table->tb6_lock);
+	if (rt->rt6i_pcpu) {
+		p = this_cpu_ptr(rt->rt6i_pcpu);
+		prev = cmpxchg(p, NULL, pcpu_rt);
+		if (prev) {
+			/* If someone did it before us, return prev instead */
+			dst_release_immediate(&pcpu_rt->dst);
+			pcpu_rt = prev;
+		}
+	} else {
+		/* rt has been removed from the fib6 tree
+		 * before we have a chance to acquire the read_lock.
+		 * In this case, don't brother to create a pcpu rt
+		 * since rt is going away anyway.  The next
+		 * dst_check() will trigger a re-lookup.
+		 */
 		dst_release_immediate(&pcpu_rt->dst);
-		/* release refcnt taken by above dst_hold() */
-		dst_release_immediate(&pcpu_rt->dst);
-		dst_hold(&prev->dst);
-		pcpu_rt = prev;
+		pcpu_rt = rt;
 	}
-
+	dst_hold(&pcpu_rt->dst);
 	rt6_dst_from_metrics_check(pcpu_rt);
+	read_unlock_bh(&table->tb6_lock);
 	return pcpu_rt;
 }
 
@@ -1168,28 +1177,19 @@ redo_rt6_select:
 		if (pcpu_rt) {
 			read_unlock_bh(&table->tb6_lock);
 		} else {
-			/* atomic_inc_not_zero() is needed when using rcu */
-			if (atomic_inc_not_zero(&rt->rt6i_ref)) {
-				/* We have to do the read_unlock first
-				 * because rt6_make_pcpu_route() may trigger
-				 * ip6_dst_gc() which will take the write_lock.
-				 *
-				 * No dst_hold() on rt is needed because grabbing
-				 * rt->rt6i_ref makes sure rt can't be released.
-				 */
-				read_unlock_bh(&table->tb6_lock);
-				pcpu_rt = rt6_make_pcpu_route(rt);
-				rt6_release(rt);
-			} else {
-				/* rt is already removed from tree */
-				read_unlock_bh(&table->tb6_lock);
-				pcpu_rt = net->ipv6.ip6_null_entry;
-				dst_hold(&pcpu_rt->dst);
-			}
+			/* We have to do the read_unlock first
+			 * because rt6_make_pcpu_route() may trigger
+			 * ip6_dst_gc() which will take the write_lock.
+			 */
+			dst_hold(&rt->dst);
+			read_unlock_bh(&table->tb6_lock);
+			pcpu_rt = rt6_make_pcpu_route(rt);
+			dst_release(&rt->dst);
 		}
 
 		trace_fib6_table_lookup(net, pcpu_rt, table->tb6_id, fl6);
 		return pcpu_rt;
+
 	}
 }
 EXPORT_SYMBOL_GPL(ip6_pol_route);