brd: remove brd_devices_mutex mutex

If brd_alloc() from brd_probe() is called before brd_alloc() from
brd_init() is called, module loading will fail with -EEXIST error.
To close this race, call __register_blkdev() just before leaving
brd_init().

Then, we can remove brd_devices_mutex mutex, for brd_device list
will no longer be accessed concurrently.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Link: https://lore.kernel.org/r/6b074af7-c165-4fab-b7da-8270a4f6f6cd@i-love.sakura.ne.jp
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Tetsuo Handa

drivers/block/brd.c

@@ -362,7 +362,6 @@ __setup("ramdisk_size=", ramdisk_size);
  * (should share code eventually).
  */
 static LIST_HEAD(brd_devices);
-static DEFINE_MUTEX(brd_devices_mutex);
 static struct dentry *brd_debugfs_dir;
 
 static int brd_alloc(int i)
@@ -372,21 +371,14 @@ static int brd_alloc(int i)
 	char buf[DISK_NAME_LEN];
 	int err = -ENOMEM;
 
-	mutex_lock(&brd_devices_mutex);
-	list_for_each_entry(brd, &brd_devices, brd_list) {
-		if (brd->brd_number == i) {
-			mutex_unlock(&brd_devices_mutex);
+	list_for_each_entry(brd, &brd_devices, brd_list)
+		if (brd->brd_number == i)
 			return -EEXIST;
-		}
-	}
 	brd = kzalloc(sizeof(*brd), GFP_KERNEL);
-	if (!brd) {
-		mutex_unlock(&brd_devices_mutex);
+	if (!brd)
 		return -ENOMEM;
-	}
 	brd->brd_number		= i;
 	list_add_tail(&brd->brd_list, &brd_devices);
-	mutex_unlock(&brd_devices_mutex);
 
 	spin_lock_init(&brd->brd_lock);
 	INIT_RADIX_TREE(&brd->brd_pages, GFP_ATOMIC);
@@ -429,9 +421,7 @@ static int brd_alloc(int i)
 out_cleanup_disk:
 	blk_cleanup_disk(disk);
 out_free_dev:
-	mutex_lock(&brd_devices_mutex);
 	list_del(&brd->brd_list);
-	mutex_unlock(&brd_devices_mutex);
 	kfree(brd);
 	return err;
 }
@@ -441,15 +431,19 @@ static void brd_probe(dev_t dev)
 	brd_alloc(MINOR(dev) / max_part);
 }
 
-static void brd_del_one(struct brd_device *brd)
+static void brd_cleanup(void)
 {
-	del_gendisk(brd->brd_disk);
-	blk_cleanup_disk(brd->brd_disk);
-	brd_free_pages(brd);
-	mutex_lock(&brd_devices_mutex);
-	list_del(&brd->brd_list);
-	mutex_unlock(&brd_devices_mutex);
-	kfree(brd);
+	struct brd_device *brd, *next;
+
+	debugfs_remove_recursive(brd_debugfs_dir);
+
+	list_for_each_entry_safe(brd, next, &brd_devices, brd_list) {
+		del_gendisk(brd->brd_disk);
+		blk_cleanup_disk(brd->brd_disk);
+		brd_free_pages(brd);
+		list_del(&brd->brd_list);
+		kfree(brd);
+	}
 }
 
 static inline void brd_check_and_reset_par(void)
@@ -473,9 +467,18 @@ static inline void brd_check_and_reset_par(void)
 
 static int __init brd_init(void)
 {
-	struct brd_device *brd, *next;
 	int err, i;
 
+	brd_check_and_reset_par();
+
+	brd_debugfs_dir = debugfs_create_dir("ramdisk_pages", NULL);
+
+	for (i = 0; i < rd_nr; i++) {
+		err = brd_alloc(i);
+		if (err)
+			goto out_free;
+	}
+
 	/*
 	 * brd module now has a feature to instantiate underlying device
 	 * structure on-demand, provided that there is an access dev node.
@@ -491,42 +494,26 @@ static int __init brd_init(void)
 	 *	dynamically.
 	 */
 
-	if (__register_blkdev(RAMDISK_MAJOR, "ramdisk", brd_probe))
-		return -EIO;
-
-	brd_check_and_reset_par();
-
-	brd_debugfs_dir = debugfs_create_dir("ramdisk_pages", NULL);
-
-	for (i = 0; i < rd_nr; i++) {
-		err = brd_alloc(i);
-		if (err)
-			goto out_free;
+	if (__register_blkdev(RAMDISK_MAJOR, "ramdisk", brd_probe)) {
+		err = -EIO;
+		goto out_free;
 	}
 
 	pr_info("brd: module loaded\n");
 	return 0;
 
 out_free:
-	unregister_blkdev(RAMDISK_MAJOR, "ramdisk");
-	debugfs_remove_recursive(brd_debugfs_dir);
-
-	list_for_each_entry_safe(brd, next, &brd_devices, brd_list)
-		brd_del_one(brd);
+	brd_cleanup();
 
 	pr_info("brd: module NOT loaded !!!\n");
 	return err;
 }
 
 static void __exit brd_exit(void)
 {
-	struct brd_device *brd, *next;
 
 	unregister_blkdev(RAMDISK_MAJOR, "ramdisk");
-	debugfs_remove_recursive(brd_debugfs_dir);
-
-	list_for_each_entry_safe(brd, next, &brd_devices, brd_list)
-		brd_del_one(brd);
+	brd_cleanup();
 
 	pr_info("brd: module unloaded\n");
 }