btrfs: do not call close_fs_devices in btrfs_rm_device

There's a subtle case where if we're removing the seed device from a
file system we need to free its private copy of the fs_devices.  However
we do not need to call close_fs_devices(), because at this point there
are no devices left to close as we've closed the last one.  The only
thing that close_fs_devices() does is decrement ->opened, which should
be 1.  We want to avoid calling close_fs_devices() here because it has a
lockdep_assert_held(&uuid_mutex), and we are going to stop holding the
uuid_mutex in this path.

So simply decrement the  ->opened counter like we should, and then clean
up like normal.  Also add a comment explaining what we're doing here as
I initially removed this code erroneously.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 0941f61d80710554685c389f09455ac0fab312d4..918ad3790791e15e2eb9e52823f13351a5a0df3f 100644 (file)
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -2211,9 +2211,17 @@ int btrfs_rm_device(struct btrfs_fs_info *fs_info, const char *device_path,
        synchronize_rcu();
        btrfs_free_device(device);
 
+       /*
+        * This can happen if cur_devices is the private seed devices list.  We
+        * cannot call close_fs_devices() here because it expects the uuid_mutex
+        * to be held, but in fact we don't need that for the private
+        * seed_devices, we can simply decrement cur_devices->opened and then
+        * remove it from our list and free the fs_devices.
+        */
        if (cur_devices->num_devices == 0) {
                list_del_init(&cur_devices->seed_list);
-               close_fs_devices(cur_devices);
+               ASSERT(cur_devices->opened == 1);
+               cur_devices->opened--;
                free_fs_devices(cur_devices);
        }