net_sched: convert idrinfo->lock from spinlock to a mutex

In commit ec3ed293e7 ("net_sched: change tcf_del_walker() to take idrinfo->lock")
we move fl_hw_destroy_tmplt() to a workqueue to avoid blocking
with the spinlock held. Unfortunately, this causes a lot of
troubles here:

1. tcf_chain_destroy() could be called right after we queue the work
   but before the work runs. This is a use-after-free.

2. The chain refcnt is already 0, we can't even just hold it again.
   We can check refcnt==1 but it is ugly.

3. The chain with refcnt 0 is still visible in its block, which means
   it could be still found and used!

4. The block has a refcnt too, we can't hold it without introducing a
   proper API either.

We can make it working but the end result is ugly. Instead of wasting
time on reviewing it, let's just convert the troubling spinlock to
a mutex, which allows us to use non-atomic allocations too.

Fixes: ec3ed293e7 ("net_sched: change tcf_del_walker() to take idrinfo->lock")
Reported-by: Ido Schimmel <idosch@idosch.org>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Vlad Buslov <vladbu@mellanox.com>
Cc: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Tested-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/act_api.h

@@ -13,7 +13,7 @@
 #include <net/netns/generic.h>
 
 struct tcf_idrinfo {
-	spinlock_t	lock;
+	struct mutex	lock;
 	struct idr	action_idr;
 };
 
@@ -117,7 +117,7 @@ int tc_action_net_init(struct tc_action_net *tn,
 	if (!tn->idrinfo)
 		return -ENOMEM;
 	tn->ops = ops;
-	spin_lock_init(&tn->idrinfo->lock);
+	mutex_init(&tn->idrinfo->lock);
 	idr_init(&tn->idrinfo->action_idr);
 	return err;
 }