btrfs: zoned: fix deadlock between metadata writeback and transaction commit

btrfs: zoned: fix deadlock between metadata writeback and transaction commit

When writing out metadata extent buffers in a zoned filesystem,
btree_writepages() holds fs_info->zoned_meta_io_lock across the whole
writeback loop, including the call to btrfs_check_meta_write_pointer() ->
check_bg_is_active().

For the tree-log block group, check_bg_is_active() may fail to activate
the zone and fall back to btrfs_zone_finish_one_bg() to free an active
zone. That path waits for the running transaction to commit while still
holding zoned_meta_io_lock, but the committer needs that same lock to
write out the tree extents, so the two tasks deadlock:

  Task A (kworker, metadata writeback)      Task B (fsstress, transaction commit)
  ------------------------------------      -------------------------------------
  wb_workfn()                               btrfs_commit_transaction(T)
   btree_writepages()                        btrfs_write_and_wait_transaction()
    btrfs_zoned_meta_io_lock()                btrfs_write_marked_extents()
    btrfs_check_meta_write_pointer()           btree_writepages()
     check_bg_is_active() [treelog_bg]          btrfs_zoned_meta_io_lock()
      btrfs_zone_finish_one_bg()               <blocks on zoned_meta_io_lock,
       btrfs_zone_finish()                      held by Task A>
        do_zone_finish()
         btrfs_inc_block_group_ro()
          btrfs_wait_for_commit()
           <blocks waiting for commit
            of transaction T, done by
            Task B>

The sibling branch in check_bg_is_active() already drops zoned_meta_io_lock
around do_zone_finish() for this exact reason. Do the same in the tree-log
branch: release the lock around btrfs_zone_finish_one_bg() and re-acquire
it afterwards. The lock only protects fs_info->active_{meta,system}_bg,
which this branch does not touch, and ctx->zoned_bg keeps a reference to
the block group across the unlock, so nothing is lost while the lock
is dropped.

This hang occasionally reproduces with fstests generic/475 on a zoned
btrfs filesystem.

Fixes: 13bb483d32ab ("btrfs: zoned: activate metadata block group on write time")
Reviewed-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>