rbd: move calls that may sleep out of spin lock range

get_user() and set_disk_ro() may allocate memory, leading to a
potential deadlock if theye are called while a spin lock is held.

Move the acquisition and release of rbd_dev->lock from rbd_ioctl()
into rbd_ioctl_set_ro(), so it can occur between get_user() and
set_disk_ro().
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Alex Elder <elder@linaro.org>

drivers/block/rbd.c

@@ -561,9 +561,12 @@ static void rbd_release(struct gendisk *disk, fmode_t mode)
 
 static int rbd_ioctl_set_ro(struct rbd_device *rbd_dev, unsigned long arg)
 {
+	int ret = 0;
 	int val;
 	bool ro;
+	bool ro_changed = false;
 
+	/* get_user() may sleep, so call it before taking rbd_dev->lock */
 	if (get_user(val, (int __user *)(arg)))
 		return -EFAULT;
 
@@ -572,12 +575,25 @@ static int rbd_ioctl_set_ro(struct rbd_device *rbd_dev, unsigned long arg)
 	if (rbd_dev->spec->snap_id != CEPH_NOSNAP && !ro)
 		return -EROFS;
 
+	spin_lock_irq(&rbd_dev->lock);
+	/* prevent others open this device */
+	if (rbd_dev->open_count > 1) {
+		ret = -EBUSY;
+		goto out;
+	}
+
 	if (rbd_dev->mapping.read_only != ro) {
 		rbd_dev->mapping.read_only = ro;
-		set_disk_ro(rbd_dev->disk, ro ? 1 : 0);
+		ro_changed = true;
 	}
 
-	return 0;
+out:
+	spin_unlock_irq(&rbd_dev->lock);
+	/* set_disk_ro() may sleep, so call it after releasing rbd_dev->lock */
+	if (ret == 0 && ro_changed)
+		set_disk_ro(rbd_dev->disk, ro ? 1 : 0);
+
+	return ret;
 }
 
 static int rbd_ioctl(struct block_device *bdev, fmode_t mode,
@@ -586,13 +602,6 @@ static int rbd_ioctl(struct block_device *bdev, fmode_t mode,
 	struct rbd_device *rbd_dev = bdev->bd_disk->private_data;
 	int ret = 0;
 
-	spin_lock_irq(&rbd_dev->lock);
-	/* prevent others open this device */
-	if (rbd_dev->open_count > 1) {
-		ret = -EBUSY;
-		goto out;
-	}
-
 	switch (cmd) {
 	case BLKROSET:
 		ret = rbd_ioctl_set_ro(rbd_dev, arg);
@@ -601,8 +610,6 @@ static int rbd_ioctl(struct block_device *bdev, fmode_t mode,
 		ret = -ENOTTY;
 	}
 
-out:
-	spin_unlock_irq(&rbd_dev->lock);
 	return ret;
 }