btrfs: fix non-empty delayed iputs list on unmount due to async workers

At close_ctree() after we have ran delayed iputs either explicitly through
calling btrfs_run_delayed_iputs() or later during the call to
btrfs_commit_super() or btrfs_error_commit_super(), we assert that the
delayed iputs list is empty.

We have (another) race where this assertion might fail because we have
queued an async write into the fs_info->workers workqueue. Here's how it
happens:

1) We are submitting a data bio for an inode that is not the data
   relocation inode, so we call btrfs_wq_submit_bio();

2) btrfs_wq_submit_bio() submits a work for the fs_info->workers queue
   that will run run_one_async_done();

3) We enter close_ctree(), flush several work queues except
   fs_info->workers, explicitly run delayed iputs with a call to
   btrfs_run_delayed_iputs() and then again shortly after by calling
   btrfs_commit_super() or btrfs_error_commit_super(), which also run
   delayed iputs;

4) run_one_async_done() is executed in the work queue, and because there
   was an IO error (bio->bi_status is not 0) it calls btrfs_bio_end_io(),
   which drops the final reference on the associated ordered extent by
   calling btrfs_put_ordered_extent() - and that adds a delayed iput for
   the inode;

5) At close_ctree() we find that after stopping the cleaner and
   transaction kthreads the delayed iputs list is not empty, failing the
   following assertion:

      ASSERT(list_empty(&fs_info->delayed_iputs));

Fix this by flushing the fs_info->workers workqueue before running delayed
iputs at close_ctree().

David reported this when running generic/648, which exercises IO error
paths by using the DM error table.

Reported-by: David Sterba <dsterba@suse.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index b0f125d8efa0..984145147716 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -4341,6 +4341,19 @@ void __cold close_ctree(struct btrfs_fs_info *fs_info)
 	btrfs_flush_workqueue(fs_info->delalloc_workers);
 
 	/*
+	 * We can have ordered extents getting their last reference dropped from
+	 * the fs_info->workers queue because for async writes for data bios we
+	 * queue a work for that queue, at btrfs_wq_submit_bio(), that runs
+	 * run_one_async_done() which calls btrfs_bio_end_io() in case the bio
+	 * has an error, and that later function can do the final
+	 * btrfs_put_ordered_extent() on the ordered extent attached to the bio,
+	 * which adds a delayed iput for the inode. So we must flush the queue
+	 * so that we don't have delayed iputs after committing the current
+	 * transaction below and stopping the cleaner and transaction kthreads.
+	 */
+	btrfs_flush_workqueue(fs_info->workers);
+
+	/*
 	 * When finishing a compressed write bio we schedule a work queue item
 	 * to finish an ordered extent - btrfs_finish_compressed_write_work()
 	 * calls btrfs_finish_ordered_extent() which in turns does a call to