Ivan Vercera was recently backporting commit
9c13cb8bb477a83b9a3c9e5a5478a4e21294a760 to a RHEL kernel, and I noticed that,
while this patch protects the tg3 driver from having its ndo_poll_controller
routine called during device initalization, it does nothing for the driver
during shutdown. I.e. it would be entirely possible to have the
ndo_poll_controller method (or subsequently the ndo_poll) routine called for a
driver in the netpoll path on CPU A while in parallel on CPU B, the ndo_close or
ndo_open routine could be called.  Given that the two latter routines tend to
initizlize and free many data structures that the former two rely on, the result
can easily be data corruption or various other crashes.  Furthermore, it seems
that this is potentially a problem with all net drivers that support netpoll,
and so this should ideally be fixed in a common path.

As Ben H Pointed out to me, we can't preform dev_open/dev_close in atomic
context, so I've come up with this solution.  I've modified the npinfo->rx_flags
to make use of bitops, so that we can atomically set/clear individual bits.
We use one of these bits to provide mutual exclusion between the netpoll polling
path and the dev_open/close paths.  I can't say Im a huge fan of returning
-EBUSY in open/close, but I like it better than busy waiting while the poll path
completes.

I've tested this here by flooding netconsole with messages on a system whos nic
driver I modfied to periodically return NETDEV_TX_BUSY, so that the netpoll tx
workqueue would be forced to send frames and poll the device.  While this was
going on I rapidly ifdown/up'ed the interface and watched for any problems.
I've not found any.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
CC: Ivan Vecera <ivecera@redhat.com>
CC: "David S. Miller" <davem@davemloft.net>
CC: Ben Hutchings <redacted>
---
 include/linux/netpoll.h | 14 +++++++++----
 net/core/dev.c          | 53 +++++++++++++++++++++++++++++++++++++++++++++++++
 net/core/netpoll.c      | 23 ++++++++++++++-------
 3 files changed, 79 insertions(+), 11 deletions(-)

diff --git a/include/linux/netpoll.h b/include/linux/netpoll.h
index f54c3bb..09ef2e5 100644
--- a/include/linux/netpoll.h
+++ b/include/linux/netpoll.h

@@ -35,10 +35,14 @@ struct netpoll {
 	struct rcu_head rcu;
 };
 
+#define NETPOLL_RX_ENABLED  1
+#define NETPOLL_RX_DROP     2
+#define NETPOLL_RX_ACTIVE   3
+
 struct netpoll_info {
 	atomic_t refcnt;
 
-	int rx_flags;
+	unsigned long flags;
 	spinlock_t rx_lock;
 	struct list_head rx_np; /* netpolls that registered an rx_hook */