commit 13f172ff26563995049abe73f6eeba828de3c09d
Author: Neil Horman <nhorman@tuxdriver.com>
Date:   Fri Apr 22 08:10:59 2011 +0000

    netconsole: fix deadlock when removing net driver that netconsole is using (v2)
    
    A deadlock was reported to me recently that occured when netconsole was being
    used in a virtual guest.  If the virtio_net driver was removed while netconsole
    was setup to use an interface that was driven by that driver, the guest
    deadlocked.  No backtrace was provided because netconsole was the only console
    configured, but it became clear pretty quickly what the problem was.  In
    netconsole_netdev_event, if we get an unregister event, we call
    __netpoll_cleanup with the target_list_lock held and irqs disabled.
    __netpoll_cleanup can, if pending netpoll packets are waiting call
    cancel_delayed_work_sync, which is a sleeping path.  the might_sleep call in
    that path gets triggered, causing a console warning to be issued.  The
    netconsole write handler of course tries to take the target_list_lock again,
    which we already hold, causing deadlock.
    
    The fix is pretty striaghtforward.  Simply drop the target_list_lock and
    re-enable irqs prior to calling __netpoll_cleanup, the re-acquire the lock, and
    restart the loop.  Confirmed by myself to fix the problem reported.
    
    Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
    CC: "David S. Miller" <davem@davemloft.net>
    Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index dfb67eb..eb41e44 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c
@@ -671,6 +671,7 @@ static int netconsole_netdev_event(struct notifier_block *this,
 		goto done;
 
 	spin_lock_irqsave(&target_list_lock, flags);
+restart:
 	list_for_each_entry(nt, &target_list, list) {
 		netconsole_target_get(nt);
 		if (nt->np.dev == dev) {
@@ -683,9 +684,16 @@ static int netconsole_netdev_event(struct notifier_block *this,
 				 * rtnl_lock already held
 				 */
 				if (nt->np.dev) {
+					spin_unlock_irqrestore(
+							      &target_list_lock,
+							      flags);
 					__netpoll_cleanup(&nt->np);
+					spin_lock_irqsave(&target_list_lock,
+							  flags);
 					dev_put(nt->np.dev);
 					nt->np.dev = NULL;
+					netconsole_target_put(nt);
+					goto restart;
 				}
 				/* Fall through */
 			case NETDEV_GOING_DOWN: