[PATCH bpf-next 1/8] flow_dissector: Don't grab update-side lock on prog detach from pre_exit

[Jakub Sitnicki <jakub@cloudflare.com>]

When there are no references to struct net left, that is check_net(net) is
false, we don't need to synchronize updates to flow_dissector prog with BPF
prog attach/detach callbacks. That allows us to pull locking up from the
shared detach routine and into the bpf prog detach callback.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
---
 net/core/flow_dissector.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index 5dceed467f64..73cc085dc2b7 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -164,22 +164,26 @@ static int flow_dissector_bpf_prog_detach(struct net *net)
 {
 	struct bpf_prog *attached;
 
-	mutex_lock(&flow_dissector_mutex);
+	/* No need for update-side lock when net is going away. */
 	attached = rcu_dereference_protected(net->flow_dissector_prog,
+					     !check_net(net) ||
 					     lockdep_is_held(&flow_dissector_mutex));
-	if (!attached) {
-		mutex_unlock(&flow_dissector_mutex);
+	if (!attached)
 		return -ENOENT;
-	}
 	RCU_INIT_POINTER(net->flow_dissector_prog, NULL);
 	bpf_prog_put(attached);
-	mutex_unlock(&flow_dissector_mutex);
 	return 0;
 }
 
 int skb_flow_dissector_bpf_prog_detach(const union bpf_attr *attr)
 {
-	return flow_dissector_bpf_prog_detach(current->nsproxy->net_ns);
+	int ret;
+
+	mutex_lock(&flow_dissector_mutex);
+	ret = flow_dissector_bpf_prog_detach(current->nsproxy->net_ns);
+	mutex_unlock(&flow_dissector_mutex);
+
+	return ret;
 }
 
 static void __net_exit flow_dissector_pernet_pre_exit(struct net *net)
-- 
2.25.4