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 <borntraeger@de.ibm.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Sainath Grandhi <sainath.grandhi@intel.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tap.c

@@ -106,7 +106,7 @@ struct major_info {
 	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;
 };
@@ -416,15 +416,15 @@ int tap_get_minor(dev_t major, struct tap_dev *tap)
 		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();
@@ -442,12 +442,12 @@ void tap_free_minor(dev_t major, struct tap_dev *tap)
 		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();
@@ -467,13 +467,13 @@ static struct tap_dev *dev_get_by_tap_file(int major, int minor)
 		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();
@@ -1244,7 +1244,7 @@ static int tap_list_add(dev_t major, const char *device_name)
 	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;