Re: [PATCH] btrfs: wait for in-flight readahead BIOs on open_ctree() error

[Teng Liu <27rabbitlt@gmail.com>]

Thanks for your review!
On 2026-03-29 17:33, Qu Wenruo wrote:
> 
> 
> This doesn't make any sense to me.
It confuses me as well when I try to reproduce the bug. The reported
claimed that btrfs_bio_counter_sub triggered a use-after-free but this
function lives under `dev-reaplce.c` which should have nothing to do
with the setting from the name.

However when I checked the function call chain: 

open_ctree()
  → btrfs_read_sys_array()          # OK — sys_chunk_array in superblock is intact
  → load_super_root(chunk_root)     # OK — reads root node, passes validation
  → btrfs_read_chunk_tree()
      → btrfs_for_each_slot()
          → readahead_tree_node_children(node)
              → for each child pointer in the internal node:
                  btrfs_readahead_node_child()
                    → btrfs_readahead_tree_block()
                      → read_extent_buffer_pages_nowait()
                        → btrfs_submit_bbio()
                          → btrfs_submit_chunk()
                            → btrfs_bio_counter_inc_blocked()  ← bio_counter++
                            → btrfs_map_block()               
                            → submit_bio()                     ← sent to USB drive

After submit_bio() sends BIO to USB drive, we continue on
read_one_dev():

open_ctree()
  → btrfs_read_sys_array()          # OK — sys_chunk_array in superblock is intact
  → load_super_root(chunk_root)     # OK — reads root node, passes validation
  → btrfs_read_chunk_tree()
      → btrfs_for_each_slot()
          → readahead_tree_node_children(node)
            → bio_coutner++ and submit_bio() send BIO to USB drive
          → read_one_dev()

This read_one_dev will return an error since the leaf block is actually
corrupted. Then open_ctree will get into error path and try to free
fs_info. 

After USB device finished BIO, it will try to decreament the counter but
the fs_info is already freed.

Any suggestions on this?


> 
> The wait and counter are all for dev-reaplce, not matching your description
> of the generic metadata readahead.
> 
> If you want to wait for all existing metadata reads, I didn't find a good
> helper, thus you will need to go through all extent buffers and wait for
> EXTENT_BUFFER_READING flags.
> 
>