[PATCH 4/4] fs: btrfs: fix a data race in btrfs_block_rsv_release()

[Jia-Ju Bai]

The functions btrfs_block_rsv_release() and
btrfs_update_delayed_refs_rsv() are concurrently executed at runtime in
the following call contexts:

Thread 1:
  btrfs_file_write_iter()
    btrfs_buffered_write()
      btrfs_delalloc_release_extents()
        btrfs_inode_rsv_release()
          __btrfs_block_rsv_release()

Thread 2:
  finish_ordered_fn()
    btrfs_finish_ordered_io()
      insert_reserved_file_extent()
        __btrfs_drop_extents()
          btrfs_free_extent()
            btrfs_add_delayed_data_ref()
              btrfs_update_delayed_refs_rsv()

In __btrfs_block_rsv_release():
  else if (... && !delayed_rsv->full)

In btrfs_update_delayed_refs_rsv():
  spin_lock(&delayed_rsv->lock);
  delayed_rsv->size += num_bytes;
  delayed_rsv->full = 0;
  spin_unlock(&delayed_rsv->lock);

Thus a data race for delayed_rsv->full can occur.
This race was found and actually reproduced by our conccurency fuzzer.

To fix this race, the spinlock delayed_rsv->lock is used to
protect the access to delayed_rsv->full in btrfs_block_rsv_release().

Signed-off-by: Jia-Ju Bai <baijiaju1990@xxxxxxxxx>
---
 fs/btrfs/block-rsv.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/block-rsv.c b/fs/btrfs/block-rsv.c
index 27efec8f7c5b..89c53a7137b4 100644
--- a/fs/btrfs/block-rsv.c
+++ b/fs/btrfs/block-rsv.c
@@ -277,6 +277,11 @@ u64 btrfs_block_rsv_release(struct btrfs_fs_info *fs_info,
 	struct btrfs_block_rsv *global_rsv = &fs_info->global_block_rsv;
 	struct btrfs_block_rsv *delayed_rsv = &fs_info->delayed_refs_rsv;
 	struct btrfs_block_rsv *target = NULL;
+	unsigned short full = 0;
+
+	spin_lock(&delayed_rsv->lock);
+	full = delayed_rsv->full;
+	spin_unlock(&delayed_rsv->lock);
 
 	/*
 	 * If we are the delayed_rsv then push to the global rsv, otherwise dump
@@ -284,7 +289,7 @@ u64 btrfs_block_rsv_release(struct btrfs_fs_info *fs_info,
 	 */
 	if (block_rsv == delayed_rsv)
 		target = global_rsv;
-	else if (block_rsv != global_rsv && !delayed_rsv->full)
+	else if (block_rsv != global_rsv && !full)
 		target = delayed_rsv;
 
 	if (target && block_rsv->space_info != target->space_info)
-- 
2.17.1