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

[Jakub Sitnicki]

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 <ja...@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