[v2,0/5] btrfs: block_group refcounting fixes

Message

Boris Burkov March 10, 2025, 8:07 p.m. UTC

We have observed a number of WARNINGs in the Meta fleet which are the
result of a block_group refcount underflowing. The refcount error
can happen at any point in the block group's lifetime, so it is hard to
conclude that we have reproduced/fixed all the bugs, I believe I have
found a few here that will hopefully improve things.

The main thrust of this patch series is that we need to take the
fs_info->unused_bgs_lock spin lock when modifying the bg_list of a
block_group. There are a number of code paths where we atomically check
that list_head for emptiness and then add/del get/put appropriately.
If any other thread messes with it in between without locking, then that
logic gets messed up. This is most obviously evident with
btrfs_mark_bg_unused.

I could imagine universally protecting bg_list's empty/not-empty nature
with a lock with smaller scope, but this is already the locking strategy
being used to synchronize reclaim/unused lists, so it seems reasonable
to just re-use it.

In addition, I attempted to simplify the refcounting logic in the
discard workfn, as the last time I fixed a bug in there, I made it far
too subtle. Hopefully this more explicit variant is easier to analyze in
the future.
--
Changelog
v2:
- fix mistaken placement of a btrfs_block_group put in the 2nd
  (locking) patch, when it ought to be in the 4th (ref-counting) patch.
- improve several commit messages with more details and using full
  function names instead of shorthand.
- add comments about over-paranoid locking.
- rename second patch to reflect that it is hardening rather than
  fixing any bugs.
- fix bad comment and variable names in btrfs_link_bg_list.


Boris Burkov (5):
  btrfs: fix bg refcount race in btrfs_create_pending_block_groups
  btrfs: harden bg->bg_list against list_del races
  btrfs: make discard_workfn block_group ref explicit
  btrfs: explicitly ref count block_group on new_bgs list
  btrfs: codify pattern for adding block_group to bg_list

 fs/btrfs/block-group.c | 57 +++++++++++++++++++++++++-----------------
 fs/btrfs/discard.c     | 34 ++++++++++++-------------
 fs/btrfs/extent-tree.c |  8 ++++++
 fs/btrfs/transaction.c | 13 ++++++++++
 4 files changed, 71 insertions(+), 41 deletions(-)