Re: [PATCH v2 04/23] netconsole: Call netpoll_cleanup() in process context

The netconsole driver currently deadlocks if a NETDEV_UNREGISTER event
is received while netconsole is in use, which in turn causes it to pin a
reference to the network device.  The first deadlock was dealt with in
3b410a31 so that we wouldn't recursively grab RTNL, but even calling
__netpoll_cleanup isn't safe to do considering that we are in atomic
context.  __netpoll_cleanup assumes it can sleep and has several
sleeping calls, such as synchronize_rcu_bh and
cancel_rearming_delayed_work.

Fix this by deferring netpoll_cleanup using scheduling work that
operates in process context.  We have to grab a reference to the
config_item in this case as we need to pin the item in place until it is
operated on.

Signed-off-by: Mike Waychison <redacted>
---
 drivers/net/netconsole.c |   55 ++++++++++++++++++++++++++++++++++++++++------
 1 files changed, 48 insertions(+), 7 deletions(-)

diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index 288a025..02ba5c4 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c

@@ -106,6 +106,7 @@ struct netconsole_target {
 #endif
 	int			np_state;
 	struct netpoll		np;
+	struct work_struct	cleanup_work;
 };
 
 #ifdef	CONFIG_NETCONSOLE_DYNAMIC

@@ -166,6 +167,22 @@ static void netconsole_target_put(struct netconsole_target *nt)
 
 #endif	/* CONFIG_NETCONSOLE_DYNAMIC */
 
+static void deferred_netpoll_cleanup(struct work_struct *work)
+{
+	struct netconsole_target *nt;
+	unsigned long flags;
+
+	nt = container_of(work, struct netconsole_target, cleanup_work);
+	netpoll_cleanup(&nt->np);
+
+	spin_lock_irqsave(&target_list_lock, flags);
+	BUG_ON(nt->np_state != NETPOLL_CLEANING);
+	nt->np_state = NETPOLL_DISABLED;
+	spin_unlock_irqrestore(&target_list_lock, flags);
+
+	netconsole_target_put(nt);
+}
+