net: ipv4: fix RCU races on dst refcounts

commit c6cffba4ffa2 (ipv4: Fix input route performance regression.)
added various fatal races with dst refcounts.

crashes happen on tcp workloads if routes are added/deleted at the same
time.

The dst_free() calls from free_fib_info_rcu() are clearly racy.

We need instead regular dst refcounting (dst_release()) and make
sure dst_release() is aware of RCU grace periods :

Add DST_RCU_FREE flag so that dst_release() respects an RCU grace period
before dst destruction for cached dst

Introduce a new inet_sk_rx_dst_set() helper, using atomic_inc_not_zero()
to make sure we dont increase a zero refcount (On a dst currently
waiting an rcu grace period before destruction)

rt_cache_route() must take a reference on the new cached route, and
release it if was not able to install it.

With this patch, my machines survive various benchmarks.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 12 deletions

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index fc1a81c..d6eabcf 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1199,11 +1199,6 @@ restart:
 	fnhe->fnhe_stamp = jiffies;
 }
 
-static inline void rt_free(struct rtable *rt)
-{
-	call_rcu_bh(&rt->dst.rcu_head, dst_rcu_free);
-}
-
 static void rt_cache_route(struct fib_nh *nh, struct rtable *rt)
 {
 	struct rtable *orig, *prev, **p = &nh->nh_rth_output;
@@ -1213,17 +1208,14 @@ static void rt_cache_route(struct fib_nh *nh, struct rtable *rt)
 
 	orig = *p;
 
+	rt->dst.flags |= DST_RCU_FREE;
+	dst_hold(&rt->dst);
 	prev = cmpxchg(p, orig, rt);
 	if (prev == orig) {
 		if (orig)
-			rt_free(orig);
+			dst_release(&orig->dst);
 	} else {
-		/* Routes we intend to cache in the FIB nexthop have
-		 * the DST_NOCACHE bit clear.  However, if we are
-		 * unsuccessful at storing this route into the cache
-		 * we really need to set it.
-		 */
-		rt->dst.flags |= DST_NOCACHE;
+		dst_release(&rt->dst);
 	}
 }