isdn: fix possible circular locking dependency

There's a circular locking dependency:

---> isdn_net_get_locked_lp
    --->lock &nd->queue_lock
    --->lock &nd->queue->xmit_lock
    .....................
    ---->unlock &nd->queue_lock

---> isdn_net_writebuf_skb (called with &nd->queue->xmit_lock locked)
    ---->isdn_net_inc_frame_cnt
         ---->isdn_net_device_busy
              ----> lock &nd->queue_lock

This will trigger lockdep warnings:

 =======================================================
 [ INFO: possible circular locking dependency detected ]
 2.6.32-rc4-testing #7
 -------------------------------------------------------
 ipppd/28379 is trying to acquire lock:
 (&netdev->queue_lock){......}, at: [<e62ad0fd>] isdn_net_device_busy+0x2c/0x74 [isdn]

 but task is already holding lock:
 (&netdev->local->xmit_lock){+.....}, at: [<e62aefc2>] isdn_net_write_super+0x3f/0x6e [isdn]

 which lock already depends on the new lock.
 .......

 We don't need to lock nd->queue->xmit_lock to protect single
isdn_net_lp_busy(). This can fix above lockdep warnings.

Reported-and-tested-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: Xiaotian Feng <xtfeng@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 3 insertions, 3 deletions

diff --git a/drivers/isdn/i4l/isdn_net.h b/drivers/isdn/i4l/isdn_net.h
index 74032d0881e..7511f08effa 100644
--- a/drivers/isdn/i4l/isdn_net.h
+++ b/drivers/isdn/i4l/isdn_net.h
@@ -83,19 +83,19 @@ static __inline__ isdn_net_local * isdn_net_get_locked_lp(isdn_net_dev *nd)
 
         spin_lock_irqsave(&nd->queue_lock, flags);
         lp = nd->queue;         /* get lp on top of queue */
-        spin_lock(&nd->queue->xmit_lock);
         while (isdn_net_lp_busy(nd->queue)) {
-                spin_unlock(&nd->queue->xmit_lock);
                 nd->queue = nd->queue->next;
                 if (nd->queue == lp) { /* not found -- should never happen */
                         lp = NULL;
                         goto errout;
                 }
-                spin_lock(&nd->queue->xmit_lock);
         }
         lp = nd->queue;
         nd->queue = nd->queue->next;
+        spin_unlock_irqrestore(&nd->queue_lock, flags);
+        spin_lock(&lp->xmit_lock);
         local_bh_disable();
+        return lp;
 errout:
         spin_unlock_irqrestore(&nd->queue_lock, flags);
         return lp;