[PATCH] tipc: fix an interrupt unsafe locking scenario

[Ying Xue <ying.xue@windriver.com>]

Commit 9faa89d4ed9d ("tipc: make function tipc_net_finalize() thread
safe") tries to make it thread safe to set node address, so it uses
node_list_lock lock to serialize the whole process of setting node
address in tipc_net_finalize(). But it causes the following interrupt
unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  rht_deferred_worker()
  rhashtable_rehash_table()
  lock(&(&ht->lock)->rlock)
			       tipc_nl_compat_doit()
                               tipc_net_finalize()
                               local_irq_disable();
                               lock(&(&tn->node_list_lock)->rlock);
                               tipc_sk_reinit()
                               rhashtable_walk_enter()
                               lock(&(&ht->lock)->rlock);
  <Interrupt>
  tipc_disc_rcv()
  tipc_node_check_dest()
  tipc_node_create()
  lock(&(&tn->node_list_lock)->rlock);

 *** DEADLOCK ***

When rhashtable_rehash_table() holds ht->lock on CPU0, it doesn't
disable BH. So if an interrupt happens after the lock, it can create
an inverse lock ordering between ht->lock and tn->node_list_lock. As
a consequence, deadlock might happen.

The reason causing the inverse lock ordering scenario above is because
the initial purpose of node_list_lock is not designed to do the
serialization of node address setting.

As cmpxchg() can guarantee CAS (compare-and-swap) process is atomic,
we use it to replace node_list_lock to ensure setting node address can
be atomically finished. It turns out the potential deadlock can be
avoided as well.

Fixes: 9faa89d4ed9d ("tipc: make function tipc_net_finalize() thread safe")
Signed-off-by: Ying Xue <ying.xue@windriver.com>
---
 net/tipc/net.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/net/tipc/net.c b/net/tipc/net.c
index a7f6964..62199cf 100644
--- a/net/tipc/net.c
+++ b/net/tipc/net.c
@@ -123,15 +123,13 @@ void tipc_net_finalize(struct net *net, u32 addr)
 {
 	struct tipc_net *tn = tipc_net(net);
 
-	spin_lock_bh(&tn->node_list_lock);
-	if (!tipc_own_addr(net)) {
+	if (!cmpxchg(&tn->node_addr, 0, addr)) {
 		tipc_set_node_addr(net, addr);
 		tipc_named_reinit(net);
 		tipc_sk_reinit(net);
 		tipc_nametbl_publish(net, TIPC_CFG_SRV, addr, addr,
 				     TIPC_CLUSTER_SCOPE, 0, addr);
 	}
-	spin_unlock_bh(&tn->node_list_lock);
 }
 
 void tipc_net_stop(struct net *net)
-- 
2.7.4


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