Another case that occurs is if a filesystem initially occupied 2
devices, then got unmounted, and a new filesystem is created, which
occupies a single device but with the same UUID as the one occupying 2
devices. In this case /sys/fs/btrfs/<uuid>/devices/<devname> will
correctly have 1 entry but /sys/fs/btrfs/<uuid>/devices/<devname> will
incorrectly has 2. This behavior is demonstrated by the following
script:

     UUID=292afefb-6e8c-4fb3-9d12-8c4ecb1f2374
     rm /tmp/d1
     rm /tmp/d2
     truncate -s 1G /tmp/d1
     truncate -s 1G /tmp/d2
     sudo losetup /dev/loop1 /tmp/d1
     sudo losetup /dev/loop2 /tmp/d2
     sudo mkfs.btrfs -U $UUID /dev/loop1 /dev/loop2
     sudo mount /dev/loop1 /mnt/btrfs1
     sudo umount /dev/loop1
     sudo losetup -d /dev/loop2
     sudo losetup -d /dev/loop1

     # create a new filesystem with only ONE loop-device; mount it
     rm /tmp/d1
     truncate -s 1G /tmp/d1
     sudo losetup /dev/loop1 /tmp/d1
     sudo mkfs.btrfs -U $UUID /dev/loop1
     sudo mount /dev/loop1 /mnt/btrfs1

Fix this by ensuring that device sysfs attributes are only added for
devices which are actually present at the time of mount.

Signed-off-by: Nikolay Borisov <redacted>
---
  fs/btrfs/sysfs.c | 21 ++++++++++++---------
  1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
index beb7f72d50b8..e2e110d7798a 100644
--- a/fs/btrfs/sysfs.c
+++ b/fs/btrfs/sysfs.c

@@ -12,6 +12,7 @@
  #include <crypto/hash.h>
  
  #include "ctree.h"
+#include "rcu-string.h"
  #include "discard.h"
  #include "disk-io.h"
  #include "send.h"

@@ -1611,9 +1612,13 @@ int btrfs_sysfs_add_device(struct btrfs_device *device)
  {
  	int ret;
  	unsigned int nofs_flag;
+	struct kobject *disk_kobj;
  	struct kobject *devices_kobj;
  	struct kobject *devinfo_kobj;
  
+	if (!device->bdev)
+		return 0;
+
  	/*
  	 * Make sure we use the fs_info::fs_devices to fetch the kobjects even
  	 * for the seed fs_devices

@@ -1625,16 +1630,14 @@ int btrfs_sysfs_add_device(struct btrfs_device *device)
  
  	nofs_flag = memalloc_nofs_save();
  
-	if (device->bdev) {
-		struct kobject *disk_kobj = bdev_kobj(device->bdev);
+	disk_kobj = bdev_kobj(device->bdev);
  
-		ret = sysfs_create_link(devices_kobj, disk_kobj, disk_kobj->name);
-		if (ret) {
-			btrfs_warn(device->fs_info,
-				"creating sysfs device link for devid %llu failed: %d",
-				device->devid, ret);
-			goto out;
-		}
+	ret = sysfs_create_link(devices_kobj, disk_kobj, disk_kobj->name);
+	if (ret) {
+		btrfs_warn(device->fs_info,
+			   "creating sysfs device link for devid %llu failed: %d",
+			   device->devid, ret);
+		goto out;
  	}
  
  	init_completion(&device->kobj_unregister);