IB/usnic: Fix potential deadlock

Acquiring the rtnl lock while holding usdev_lock could result in a
deadlock.

For example:

usnic_ib_query_port()
| mutex_lock(&us_ibdev->usdev_lock)
| ib_get_eth_speed()
| rtnl_lock()

rtnl_lock()
| usnic_ib_netdevice_event()
| mutex_lock(&us_ibdev->usdev_lock)

This commit moves the usdev_lock acquisition after the rtnl lock has been
released.

This is safe to do because usdev_lock is not protecting anything being
accessed in ib_get_eth_speed(). Hence, the correct order of holding locks
(rtnl -> usdev_lock) is not violated.

Signed-off-by: Parvi Kaustubhi <[e-mail]>
Signed-off-by: Jason Gunthorpe <[e-mail]>

drivers/infiniband/hw/usnic/usnic_ib_verbs.c

···
 
 	usnic_dbg("\n");
 
-	mutex_lock(&us_ibdev->usdev_lock);
 	if (ib_get_eth_speed(ibdev, port, &props->active_speed,
-			     &props->active_width)) {
-		mutex_unlock(&us_ibdev->usdev_lock);
+			     &props->active_width))
 		return -EINVAL;
-	}
 
+	/*
+	 * usdev_lock is acquired after (and not before) ib_get_eth_speed call
+	 * because acquiring rtnl_lock in ib_get_eth_speed, while holding
+	 * usdev_lock could lead to a deadlock.
+	 */
+	mutex_lock(&us_ibdev->usdev_lock);
 	/* props being zeroed by the caller, avoid zeroing it here */
 
 	props->lid = 0;