net/sched: add delete_empty() to filters and use it in cls_flower

Revert "net/sched: cls_u32: fix refcount leak in the error path of
u32_change()", and fix the u32 refcount leak in a more generic way that
preserves the semantic of rule dumping.
On tc filters that don't support lockless insertion/removal, there is no
need to guard against concurrent insertion when a removal is in progress.
Therefore, for most of them we can avoid a full walk() when deleting, and
just decrease the refcount, like it was done on older Linux kernels.
This fixes situations where walk() was wrongly detecting a non-empty
filter, like it happened with cls_u32 in the error path of change(), thus
leading to failures in the following tdc selftests:

 6aa7: (filter, u32) Add/Replace u32 with source match and invalid indev
 6658: (filter, u32) Add/Replace u32 with custom hash table and invalid handle
 74c2: (filter, u32) Add/Replace u32 filter with invalid hash table id

On cls_flower, and on (future) lockless filters, this check is necessary:
move all the check_empty() logic in a callback so that each filter
can have its own implementation. For cls_flower, it's sufficient to check
if no IDRs have been allocated.

This reverts commit 275c44aa.

Changes since v1:
 - document the need for delete_empty() when TCF_PROTO_OPS_DOIT_UNLOCKED
   is used, thanks to Vlad Buslov
 - implement delete_empty() without doing fl_walk(), thanks to Vlad Buslov
 - squash revert and new fix in a single patch, to be nice with bisect
   tests that run tdc on u32 filter, thanks to Dave Miller

Fixes: 275c44aa ("net/sched: cls_u32: fix refcount leak in the error path of u32_change()")
Fixes: 6676d5e4 ("net: sched: set dedicated tcf_walker flag when tp is empty")
Suggested-by: NJamal Hadi Salim <jhs@mojatatu.com>
Suggested-by: NVlad Buslov <vladbu@mellanox.com>
Signed-off-by: NDavide Caratti <dcaratti@redhat.com>
Reviewed-by: NVlad Buslov <vladbu@mellanox.com>
Tested-by: NJamal Hadi Salim <jhs@mojatatu.com>
Acked-by: NJamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/sch_generic.h

@@ -308,6 +308,7 @@ struct tcf_proto_ops {
 	int			(*delete)(struct tcf_proto *tp, void *arg,
 					  bool *last, bool rtnl_held,
 					  struct netlink_ext_ack *);
+	bool			(*delete_empty)(struct tcf_proto *tp);
 	void			(*walk)(struct tcf_proto *tp,
 					struct tcf_walker *arg, bool rtnl_held);
 	int			(*reoffload)(struct tcf_proto *tp, bool add,
@@ -336,6 +337,10 @@ struct tcf_proto_ops {
 	int			flags;
 };
 
+/* Classifiers setting TCF_PROTO_OPS_DOIT_UNLOCKED in tcf_proto_ops->flags
+ * are expected to implement tcf_proto_ops->delete_empty(), otherwise race
+ * conditions can occur when filters are inserted/deleted simultaneously.
+ */
 enum tcf_proto_ops_flags {
 	TCF_PROTO_OPS_DOIT_UNLOCKED = 1,
 };

net/sched/cls_api.c

@@ -308,33 +308,12 @@ static void tcf_proto_put(struct tcf_proto *tp, bool rtnl_held,
 		tcf_proto_destroy(tp, rtnl_held, true, extack);
 }
 
-static int walker_check_empty(struct tcf_proto *tp, void *fh,
-			      struct tcf_walker *arg)
+static bool tcf_proto_check_delete(struct tcf_proto *tp)
 {
-	if (fh) {
-		arg->nonempty = true;
-		return -1;
-	}
-	return 0;
-}
-
-static bool tcf_proto_is_empty(struct tcf_proto *tp, bool rtnl_held)
-{
-	struct tcf_walker walker = { .fn = walker_check_empty, };
-
-	if (tp->ops->walk) {
-		tp->ops->walk(tp, &walker, rtnl_held);
-		return !walker.nonempty;
-	}
-	return true;
-}
+	if (tp->ops->delete_empty)
+		return tp->ops->delete_empty(tp);
 
-static bool tcf_proto_check_delete(struct tcf_proto *tp, bool rtnl_held)
-{
-	spin_lock(&tp->lock);
-	if (tcf_proto_is_empty(tp, rtnl_held))
-		tp->deleting = true;
-	spin_unlock(&tp->lock);
+	tp->deleting = true;
 	return tp->deleting;
 }
 
@@ -1751,7 +1730,7 @@ static void tcf_chain_tp_delete_empty(struct tcf_chain *chain,
 	 * concurrently.
 	 * Mark tp for deletion if it is empty.
 	 */
-	if (!tp_iter || !tcf_proto_check_delete(tp, rtnl_held)) {
+	if (!tp_iter || !tcf_proto_check_delete(tp)) {
 		mutex_unlock(&chain->filter_chain_lock);
 		return;
 	}

net/sched/cls_flower.c

@@ -2773,6 +2773,17 @@ static void fl_bind_class(void *fh, u32 classid, unsigned long cl)
 		f->res.class = cl;
 }
 
+static bool fl_delete_empty(struct tcf_proto *tp)
+{
+	struct cls_fl_head *head = fl_head_dereference(tp);
+
+	spin_lock(&tp->lock);
+	tp->deleting = idr_is_empty(&head->handle_idr);
+	spin_unlock(&tp->lock);
+
+	return tp->deleting;
+}
+
 static struct tcf_proto_ops cls_fl_ops __read_mostly = {
 	.kind		= "flower",
 	.classify	= fl_classify,
@@ -2782,6 +2793,7 @@ static struct tcf_proto_ops cls_fl_ops __read_mostly = {
 	.put		= fl_put,
 	.change		= fl_change,
 	.delete		= fl_delete,
+	.delete_empty	= fl_delete_empty,
 	.walk		= fl_walk,
 	.reoffload	= fl_reoffload,
 	.hw_add		= fl_hw_add,

net/sched/cls_u32.c

@@ -1108,33 +1108,10 @@ static int u32_change(struct net *net, struct sk_buff *in_skb,
 	return err;
 }
 
-static bool u32_hnode_empty(struct tc_u_hnode *ht, bool *non_root_ht)
-{
-	int i;
-
-	if (!ht)
-		return true;
-	if (!ht->is_root) {
-		*non_root_ht = true;
-		return false;
-	}
-	if (*non_root_ht)
-		return false;
-	if (ht->refcnt < 2)
-		return true;
-
-	for (i = 0; i <= ht->divisor; i++) {
-		if (rtnl_dereference(ht->ht[i]))
-			return false;
-	}
-	return true;
-}
-
 static void u32_walk(struct tcf_proto *tp, struct tcf_walker *arg,
 		     bool rtnl_held)
 {
 	struct tc_u_common *tp_c = tp->data;
-	bool non_root_ht = false;
 	struct tc_u_hnode *ht;
 	struct tc_u_knode *n;
 	unsigned int h;
@@ -1147,8 +1124,6 @@ static void u32_walk(struct tcf_proto *tp, struct tcf_walker *arg,
 	     ht = rtnl_dereference(ht->next)) {
 		if (ht->prio != tp->prio)
 			continue;
-		if (u32_hnode_empty(ht, &non_root_ht))
-			return;
 		if (arg->count >= arg->skip) {
 			if (arg->fn(tp, ht, arg) < 0) {
 				arg->stop = 1;