[PATCH v2] btrfs: fix rw device counting in __btrfs_free_extra_devids

[Desmond Cheong Zhi Xi]

When removing a writeable device in __btrfs_free_extra_devids, the rw
device count should be decremented.

This error was caught by Syzbot which reported a warning in
close_fs_devices because fs_devices->rw_devices was not 0 after
closing all devices. Here is the call trace that was observed:

  btrfs_mount_root():
    btrfs_scan_one_device():
      device_list_add();   <---------------- device added
    btrfs_open_devices():
      open_fs_devices():
        btrfs_open_one_device();   <-------- writable device opened,
	                                     rw device count ++
    btrfs_fill_super():
      open_ctree():
        btrfs_free_extra_devids():
	  __btrfs_free_extra_devids();  <--- writable device removed,
	                              rw device count not decremented
	  fail_tree_roots:
	    btrfs_close_devices():
	      close_fs_devices();   <------- rw device count off by 1

As a note, prior to commit cf89af146b7e ("btrfs: dev-replace: fail
mount if we don't have replace item with target device"), rw_devices
was decremented on removing a writable device in
__btrfs_free_extra_devids only if the BTRFS_DEV_STATE_REPLACE_TGT bit
was not set for the device. However, this check does not need to be
reinstated as it is now redundant and incorrect.

In __btrfs_free_extra_devids, we skip removing the device if it is the
target for replacement. This is done by checking whether device->devid
== BTRFS_DEV_REPLACE_DEVID. Since BTRFS_DEV_STATE_REPLACE_TGT is set
only on the device with devid BTRFS_DEV_REPLACE_DEVID, no devices
should have the BTRFS_DEV_STATE_REPLACE_TGT bit set after the check,
and so it's redundant to test for that bit.

Additionally, following commit 82372bc816d7 ("Btrfs: make
the logic of source device removing more clear"), rw_devices is
incremented whenever a writeable device is added to the alloc
list (including the target device in btrfs_dev_replace_finishing), so
all removals of writable devices from the alloc list should also be
accompanied by a decrement to rw_devices.

Fixes: cf89af146b7e ("btrfs: dev-replace: fail mount if we don't have replace item with target device")
Reported-by: syzbot+a70e2ad0879f160b9217@xxxxxxxxxxxxxxxxxxxxxxxxx
Tested-by: syzbot+a70e2ad0879f160b9217@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
Reviewed-by: Anand Jain <anand.jain@xxxxxxxxxx>
---
 fs/btrfs/volumes.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 807502cd6510..916c25371658 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -1078,6 +1078,7 @@ static void __btrfs_free_extra_devids(struct btrfs_fs_devices *fs_devices,
 		if (test_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state)) {
 			list_del_init(&device->dev_alloc_list);
 			clear_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state);
+			fs_devices->rw_devices--;
 		}
 		list_del_init(&device->dev_list);
 		fs_devices->num_devices--;
-- 
2.25.1