btrfs: fix hang during unmount when stopping block group reclaim worker

During early unmount, at close_ctree(), we try to stop the block group
reclaim task with cancel_work_sync(), but that may hang if the block group
reclaim task is currently at btrfs_relocate_block_group() waiting for the
flag BTRFS_FS_UNFINISHED_DROPS to be cleared from fs_info->flags. During
unmount we only clear that flag later, after trying to stop the block
group reclaim task.

Fix that by clearing BTRFS_FS_UNFINISHED_DROPS before trying to stop the
block group reclaim task and after setting BTRFS_FS_CLOSING_START, so that
if the reclaim task is waiting on that bit, it will stop immediately after
being woken, because it sees the filesystem is closing (with a call to
btrfs_fs_closing()), and then returns immediately with -EINTR.

Fixes: 31e70e52 ("btrfs: fix hang during unmount when block group reclaim task is running")
CC: stable@vger.kernel.org # 5.15+
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -4476,6 +4476,17 @@ void __cold close_ctree(struct btrfs_fs_info *fs_info)
 
 	set_bit(BTRFS_FS_CLOSING_START, &fs_info->flags);
 
+	/*
+	 * If we had UNFINISHED_DROPS we could still be processing them, so
+	 * clear that bit and wake up relocation so it can stop.
+	 * We must do this before stopping the block group reclaim task, because
+	 * at btrfs_relocate_block_group() we wait for this bit, and after the
+	 * wait we stop with -EINTR if btrfs_fs_closing() returns non-zero - we
+	 * have just set BTRFS_FS_CLOSING_START, so btrfs_fs_closing() will
+	 * return 1.
+	 */
+	btrfs_wake_unfinished_drop(fs_info);
+
 	/*
 	 * We may have the reclaim task running and relocating a data block group,
 	 * in which case it may create delayed iputs. So stop it before we park
@@ -4494,12 +4505,6 @@ void __cold close_ctree(struct btrfs_fs_info *fs_info)
 	 */
 	kthread_park(fs_info->cleaner_kthread);
 
-	/*
-	 * If we had UNFINISHED_DROPS we could still be processing them, so
-	 * clear that bit and wake up relocation so it can stop.
-	 */
-	btrfs_wake_unfinished_drop(fs_info);
-
 	/* wait for the qgroup rescan worker to stop */
 	btrfs_qgroup_wait_for_completion(fs_info, false);