netfilter: flowtable: Add pending bit for offload work

Gc step can queue offloaded flow del work or stats work.
Those work items can race each other and a flow could be freed
before the stats work is executed and querying it.
To avoid that, add a pending bit that if a work exists for a flow
don't queue another work for it.
This will also avoid adding multiple stats works in case stats work
didn't complete but gc step started again.

Signed-off-by: Paul Blakey <paulb@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/nf_flow_table.h

@@ -127,6 +127,7 @@ enum nf_flow_flags {
 	NF_FLOW_HW_DYING,
 	NF_FLOW_HW_DEAD,
 	NF_FLOW_HW_REFRESH,
+	NF_FLOW_HW_PENDING,
 };
 
 enum flow_offload_type {

net/netfilter/nf_flow_table_offload.c

@@ -817,6 +817,7 @@ static void flow_offload_work_handler(struct work_struct *work)
 			WARN_ON_ONCE(1);
 	}
 
+	clear_bit(NF_FLOW_HW_PENDING, &offload->flow->flags);
 	kfree(offload);
 }
 
@@ -831,10 +832,15 @@ nf_flow_offload_work_alloc(struct nf_flowtable *flowtable,
 {
 	struct flow_offload_work *offload;
 
-	offload = kmalloc(sizeof(struct flow_offload_work), GFP_ATOMIC);
-	if (!offload)
+	if (test_and_set_bit(NF_FLOW_HW_PENDING, &flow->flags))
 		return NULL;
 
+	offload = kmalloc(sizeof(struct flow_offload_work), GFP_ATOMIC);
+	if (!offload) {
+		clear_bit(NF_FLOW_HW_PENDING, &flow->flags);
+		return NULL;
+	}
+
 	offload->cmd = cmd;
 	offload->flow = flow;
 	offload->priority = flowtable->priority;