fs: Convert unnamed_dev_ida to new API

The new API is much easier for this user.  Also add kerneldoc for
get_anon_bdev().
Signed-off-by: Matthew Wilcox <willy@infradead.org>

fs/super.c

@@ -978,58 +978,42 @@ void emergency_thaw_all(void)
 	}
 }
 
-/*
- * Unnamed block devices are dummy devices used by virtual
- * filesystems which don't use real block-devices.  -- jrs
- */
-
 static DEFINE_IDA(unnamed_dev_ida);
-static DEFINE_SPINLOCK(unnamed_dev_lock);/* protects the above */
-/* Many userspace utilities consider an FSID of 0 invalid.
- * Always return at least 1 from get_anon_bdev.
- */
-static int unnamed_dev_start = 1;
 
+/**
+ * get_anon_bdev - Allocate a block device for filesystems which don't have one.
+ * @p: Pointer to a dev_t.
+ *
+ * Filesystems which don't use real block devices can call this function
+ * to allocate a virtual block device.
+ *
+ * Context: Any context.  Frequently called while holding sb_lock.
+ * Return: 0 on success, -EMFILE if there are no anonymous bdevs left
+ * or -ENOMEM if memory allocation failed.
+ */
 int get_anon_bdev(dev_t *p)
 {
 	int dev;
-	int error;
 
- retry:
-	if (ida_pre_get(&unnamed_dev_ida, GFP_ATOMIC) == 0)
-		return -ENOMEM;
-	spin_lock(&unnamed_dev_lock);
-	error = ida_get_new_above(&unnamed_dev_ida, unnamed_dev_start, &dev);
-	if (!error)
-		unnamed_dev_start = dev + 1;
-	spin_unlock(&unnamed_dev_lock);
-	if (error == -EAGAIN)
-		/* We raced and lost with another CPU. */
-		goto retry;
-	else if (error)
-		return -EAGAIN;
-
-	if (dev >= (1 << MINORBITS)) {
-		spin_lock(&unnamed_dev_lock);
-		ida_remove(&unnamed_dev_ida, dev);
-		if (unnamed_dev_start > dev)
-			unnamed_dev_start = dev;
-		spin_unlock(&unnamed_dev_lock);
-		return -EMFILE;
-	}
-	*p = MKDEV(0, dev & MINORMASK);
+	/*
+	 * Many userspace utilities consider an FSID of 0 invalid.
+	 * Always return at least 1 from get_anon_bdev.
+	 */
+	dev = ida_alloc_range(&unnamed_dev_ida, 1, (1 << MINORBITS) - 1,
+			GFP_ATOMIC);
+	if (dev == -ENOSPC)
+		dev = -EMFILE;
+	if (dev < 0)
+		return dev;
+
+	*p = MKDEV(0, dev);
 	return 0;
 }
 EXPORT_SYMBOL(get_anon_bdev);
 
 void free_anon_bdev(dev_t dev)
 {
-	int slot = MINOR(dev);
-	spin_lock(&unnamed_dev_lock);
-	ida_remove(&unnamed_dev_ida, slot);
-	if (slot < unnamed_dev_start)
-		unnamed_dev_start = slot;
-	spin_unlock(&unnamed_dev_lock);
+	ida_free(&unnamed_dev_ida, MINOR(dev));
 }
 EXPORT_SYMBOL(free_anon_bdev);
 
@@ -1037,7 +1021,6 @@ int set_anon_super(struct super_block *s, void *data)
 {
 	return get_anon_bdev(&s->s_dev);
 }
-
 EXPORT_SYMBOL(set_anon_super);
 
 void kill_anon_super(struct super_block *sb)
@@ -1046,7 +1029,6 @@ void kill_anon_super(struct super_block *sb)
 	generic_shutdown_super(sb);
 	free_anon_bdev(dev);
 }
-
 EXPORT_SYMBOL(kill_anon_super);
 
 void kill_litter_super(struct super_block *sb)
@@ -1055,7 +1037,6 @@ void kill_litter_super(struct super_block *sb)
 		d_genocide(sb->s_root);
 	kill_anon_super(sb);
 }
-
 EXPORT_SYMBOL(kill_litter_super);
 
 static int ns_test_super(struct super_block *sb, void *data)