net/sched: Use nested-BH locking for sch_frag_data_storage

sch_frag_data_storage is a per-CPU variable and relies on disabled BH
for its locking. Without per-CPU locking in local_bh_disable() on
PREEMPT_RT this data structure requires explicit locking.

Add local_lock_t to the struct and use local_lock_nested_bh() for locking.
This change adds only lockdep coverage and does not alter the functional
behaviour for !PREEMPT_RT.

Cc: Jamal Hadi Salim <[email protected]>
Cc: Cong Wang <[email protected]>
Cc: Jiri Pirko <[email protected]>
Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>

Author: Sebastian Andrzej Siewior

net/sched/sch_frag.c

@@ -16,14 +16,18 @@ struct sch_frag_data {
 	unsigned int l2_len;
 	u8 l2_data[VLAN_ETH_HLEN];
 	int (*xmit)(struct sk_buff *skb);
+	local_lock_t bh_lock;
 };
 
-static DEFINE_PER_CPU(struct sch_frag_data, sch_frag_data_storage);
+static DEFINE_PER_CPU(struct sch_frag_data, sch_frag_data_storage) = {
+	.bh_lock = INIT_LOCAL_LOCK(bh_lock),
+};
 
 static int sch_frag_xmit(struct net *net, struct sock *sk, struct sk_buff *skb)
 {
 	struct sch_frag_data *data = this_cpu_ptr(&sch_frag_data_storage);
 
+	lockdep_assert_held(&data->bh_lock);
 	if (skb_cow_head(skb, data->l2_len) < 0) {
 		kfree_skb(skb);
 		return -ENOMEM;
@@ -95,6 +99,7 @@ static int sch_fragment(struct net *net, struct sk_buff *skb,
 		struct rtable sch_frag_rt = { 0 };
 		unsigned long orig_dst;
 
+		local_lock_nested_bh(&sch_frag_data_storage.bh_lock);
 		sch_frag_prepare_frag(skb, xmit);
 		dst_init(&sch_frag_rt.dst, &sch_frag_dst_ops, NULL,
 			 DST_OBSOLETE_NONE, DST_NOCOUNT);
@@ -105,11 +110,13 @@ static int sch_fragment(struct net *net, struct sk_buff *skb,
 		IPCB(skb)->frag_max_size = mru;
 
 		ret = ip_do_fragment(net, skb->sk, skb, sch_frag_xmit);
+		local_unlock_nested_bh(&sch_frag_data_storage.bh_lock);
 		refdst_drop(orig_dst);
 	} else if (skb_protocol(skb, true) == htons(ETH_P_IPV6)) {
 		unsigned long orig_dst;
 		struct rt6_info sch_frag_rt;
 
+		local_lock_nested_bh(&sch_frag_data_storage.bh_lock);
 		sch_frag_prepare_frag(skb, xmit);
 		memset(&sch_frag_rt, 0, sizeof(sch_frag_rt));
 		dst_init(&sch_frag_rt.dst, &sch_frag_dst_ops, NULL,
@@ -122,6 +129,7 @@ static int sch_fragment(struct net *net, struct sk_buff *skb,
 
 		ret = ipv6_stub->ipv6_fragment(net, skb->sk, skb,
 					       sch_frag_xmit);
+		local_unlock_nested_bh(&sch_frag_data_storage.bh_lock);
 		refdst_drop(orig_dst);
 	} else {
 		net_warn_ratelimited("Fail frag %s: eth=%x, MRU=%d, MTU=%d\n",