tap: convert a mutex to a spinlock

We are not allowed to block on the RCU reader side, so can't
just hold the mutex as before. As a quick fix, convert it to
a spinlock.

Fixes: d9f1f61c0801 ("tap: Extending tap device create/destroy APIs")
Reported-by: Christian Borntraeger <[e-mail]>
Tested-by: Christian Borntraeger <[e-mail]>
Cc: Sainath Grandhi <[e-mail]>
Signed-off-by: Cong Wang <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/tap.c

···
 	struct rcu_head rcu;
 	dev_t major;
 	struct idr minor_idr;
-	struct mutex minor_lock;
+	spinlock_t minor_lock;
 	const char *device_name;
 	struct list_head next;
 };
···
 		goto unlock;
 	}
 
-	mutex_lock(&tap_major->minor_lock);
-	retval = idr_alloc(&tap_major->minor_idr, tap, 1, TAP_NUM_DEVS, GFP_KERNEL);
+	spin_lock(&tap_major->minor_lock);
+	retval = idr_alloc(&tap_major->minor_idr, tap, 1, TAP_NUM_DEVS, GFP_ATOMIC);
 	if (retval >= 0) {
 		tap->minor = retval;
 	} else if (retval == -ENOSPC) {
 		netdev_err(tap->dev, "Too many tap devices\n");
 		retval = -EINVAL;
 	}
-	mutex_unlock(&tap_major->minor_lock);
+	spin_unlock(&tap_major->minor_lock);
 
 unlock:
 	rcu_read_unlock();
···
 		goto unlock;
 	}
 
-	mutex_lock(&tap_major->minor_lock);
+	spin_lock(&tap_major->minor_lock);
 	if (tap->minor) {
 		idr_remove(&tap_major->minor_idr, tap->minor);
 		tap->minor = 0;
 	}
-	mutex_unlock(&tap_major->minor_lock);
+	spin_unlock(&tap_major->minor_lock);
 
 unlock:
 	rcu_read_unlock();
···
 		goto unlock;
 	}
 
-	mutex_lock(&tap_major->minor_lock);
+	spin_lock(&tap_major->minor_lock);
 	tap = idr_find(&tap_major->minor_idr, minor);
 	if (tap) {
 		dev = tap->dev;
 		dev_hold(dev);
 	}
-	mutex_unlock(&tap_major->minor_lock);
+	spin_unlock(&tap_major->minor_lock);
 
 unlock:
 	rcu_read_unlock();
···
 	tap_major->major = MAJOR(major);
 
 	idr_init(&tap_major->minor_idr);
-	mutex_init(&tap_major->minor_lock);
+	spin_lock_init(&tap_major->minor_lock);
 
 	tap_major->device_name = device_name;