[PATCH v2 1/7] btrfs: do not call close_fs_devices in btrfs_rm_device

[Josef Bacik <josef@toxicpanda.com>]

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 add an assert for the ->opened counter and simply decrement it 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.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/volumes.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 86846d6e58d0..5217b93172b4 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -2200,9 +2200,17 @@ 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->open_devices == 0) {
+		ASSERT(cur_devices->opened == 1);
 		list_del_init(&cur_devices->seed_list);
-		close_fs_devices(cur_devices);
+		cur_devices->opened--;
 		free_fs_devices(cur_devices);
 	}
 
-- 
2.26.3