Re: 3.16-rc3: PV guest INFO: possible recursive locking detected netback_changed+0xecb/0x1100

[David Vrabel <david.vrabel@citrix.com>]

On 01/07/14 21:29, Sander Eikelenboom wrote:
> Hi,
> 
> In one of my PV guest i got the lockdep warning below on boot.

Thanks. The locking in xennet_connect() is a bit "loopy".  I think it is
actually safe because these are locks for different queues but the lock
annotations are missing.

However, I don't think this lock nesting is necessary and the following
both eliminates the lockdep warning and makes the locking simpler.

8<--------------------
xen-netfront: don't nest queue locks in xennet_connect()

The nesting of the per-queue rx_lock and tx_lock in xennet_connect()
is confusing to both humans and lockdep.  The locking is safe because
this is the only place where the locks are nested in this way but
lockdep still warns.

Instead of adding the missing lockdep annotations, refactor the
locking to avoid the confusing nesting.  This is still safe, because
the xenbus connection state changes are all serialized by the xenwatch
thread.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reported-by: Sander Eikelenboom <linux@eikelenboom.it>
---
 drivers/net/xen-netfront.c |   11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index 2ccb4a0..837c32dc 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -2076,6 +2076,9 @@ static int xennet_connect(struct net_device *dev)
 		}

 		queue->rx.req_prod_pvt = requeue_idx;
+
+		spin_unlock_irq(&queue->tx_lock);
+		spin_unlock_bh(&queue->rx_lock);
 	}

 	/*
@@ -2087,13 +2090,17 @@ static int xennet_connect(struct net_device *dev)
 	netif_carrier_on(np->netdev);
 	for (j = 0; j < num_queues; ++j) {
 		queue = &np->queues[j];
+
 		notify_remote_via_irq(queue->tx_irq);
 		if (queue->tx_irq != queue->rx_irq)
 			notify_remote_via_irq(queue->rx_irq);
-		xennet_tx_buf_gc(queue);
-		xennet_alloc_rx_buffers(queue);

+		spin_lock_irq(&queue->tx_lock);
+		xennet_tx_buf_gc(queue);
 		spin_unlock_irq(&queue->tx_lock);
+
+		spin_lock_bh(&queue->rx_lock);
+		xennet_alloc_rx_buffers(queue);
 		spin_unlock_bh(&queue->rx_lock);
 	}

-- 
1.7.10.4