netpoll: Fix __netpoll_rcu_free so that it can hold the rtnl lock

__netpoll_rcu_free is used to free netpoll structures when the rtnl_lock is
already held.  The mechanism is used to asynchronously call __netpoll_cleanup
outside of the holding of the rtnl_lock, so as to avoid deadlock.
Unfortunately, __netpoll_cleanup modifies pointers (dev->np), which means the
rtnl_lock must be held while calling it.  Further, it cannot be held, because
rcu callbacks may be issued in softirq contexts, which cannot sleep.

Fix this by converting the rcu callback to a work queue that is guaranteed to
get scheduled in process context, so that we can hold the rtnl properly while
calling __netpoll_cleanup

Tested successfully by myself.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
CC: "David S. Miller" <davem@davemloft.net>
CC: Cong Wang <amwang@redhat.com>
CC: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 10 insertions, 6 deletions

diff --git a/net/core/netpoll.c b/net/core/netpoll.c
index edcd9ad95304..c536474e2260 100644
--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c
@@ -61,6 +61,7 @@ static struct srcu_struct netpoll_srcu;
 
 static void zap_completion_queue(void);
 static void netpoll_neigh_reply(struct sk_buff *skb, struct netpoll_info *npinfo);
+static void netpoll_async_cleanup(struct work_struct *work);
 
 static unsigned int carrier_timeout = 4;
 module_param(carrier_timeout, uint, 0644);
@@ -1020,6 +1021,7 @@ int __netpoll_setup(struct netpoll *np, struct net_device *ndev, gfp_t gfp)
 
         np->dev = ndev;
         strlcpy(np->dev_name, ndev->name, IFNAMSIZ);
+        INIT_WORK(&np->cleanup_work, netpoll_async_cleanup);
 
         if ((ndev->priv_flags & IFF_DISABLE_NETPOLL) ||
             !ndev->netdev_ops->ndo_poll_controller) {
@@ -1255,25 +1257,27 @@ void __netpoll_cleanup(struct netpoll *np)
                 if (ops->ndo_netpoll_cleanup)
                         ops->ndo_netpoll_cleanup(np->dev);
 
-                RCU_INIT_POINTER(np->dev->npinfo, NULL);
+                rcu_assign_pointer(np->dev->npinfo, NULL);
                 call_rcu_bh(&npinfo->rcu, rcu_cleanup_netpoll_info);
         }
 }
 EXPORT_SYMBOL_GPL(__netpoll_cleanup);
 
-static void rcu_cleanup_netpoll(struct rcu_head *rcu_head)
+static void netpoll_async_cleanup(struct work_struct *work)
 {
-        struct netpoll *np = container_of(rcu_head, struct netpoll, rcu);
+        struct netpoll *np = container_of(work, struct netpoll, cleanup_work);
 
+        rtnl_lock();
         __netpoll_cleanup(np);
+        rtnl_unlock();
         kfree(np);
 }
 
-void __netpoll_free_rcu(struct netpoll *np)
+void __netpoll_free_async(struct netpoll *np)
 {
-        call_rcu_bh(&np->rcu, rcu_cleanup_netpoll);
+        schedule_work(&np->cleanup_work);
 }
-EXPORT_SYMBOL_GPL(__netpoll_free_rcu);
+EXPORT_SYMBOL_GPL(__netpoll_free_async);
 
 void netpoll_cleanup(struct netpoll *np)
 {