The dedup reference is a special kind of delayed refs, and the delayed refs
are batched to be processed later.
If we find a matched dedup extent, then we queue an ADD delayed ref on it during
endio work, but there is already a DROP delayed ref queued there,
t1 t2 t3
->writepage commit transaction
->run_delalloc_dedup
find_dedup
------------------------------------------------------------------------------
process_delayed refs
add ordered extent
submit pages
finish ordered io
insert file extents
queue delayed refs
queue dedup ref
This senario ends up with a crash because we're going to insert a ref on
a deleted extent.
To avoid the race, we need to wait for processing delayed refs before finding
matched dedup extents.
Signed-off-by: Liu Bo <bo.li....@oracle.com>
---
fs/btrfs/file-item.c | 20 ++++++++++++++++++++
1 files changed, 20 insertions(+), 0 deletions(-)
diff --git a/fs/btrfs/file-item.c b/fs/btrfs/file-item.c
index ddb489e..8933e13 100644
--- a/fs/btrfs/file-item.c
+++ b/fs/btrfs/file-item.c
@@ -893,6 +893,8 @@ btrfs_find_dedup_extent(struct btrfs_root *root, struct
btrfs_dedup_hash *hash)
struct extent_buffer *leaf;
struct btrfs_root *dedup_root;
struct btrfs_dedup_item *item;
+ struct btrfs_delayed_ref_root *delayed_refs;
+ struct btrfs_trans_handle *trans;
u64 hash_value;
u64 length;
u64 dedup_size;
@@ -911,6 +913,24 @@ btrfs_find_dedup_extent(struct btrfs_root *root, struct
btrfs_dedup_hash *hash)
}
dedup_root = root->fs_info->dedup_root;
+ /*
+ * This is for serializing the dedup reference add/remove,
+ * the dedup reference is one of delayed refs, so it's likely
+ * we find the dedup extent here but there is already a DROP ref
+ * on it, and this ends up that we insert a ref on a deleted
+ * extent and get crash.
+ * Therefore, before finding matched dedup extents, we should
+ * wait for delayed_ref running's finish.
+ */
+ trans = btrfs_join_transaction(root);
+ if (!IS_ERR(trans)) {
+ delayed_refs = &trans->transaction->delayed_refs;
+ if (delayed_refs &&
atomic_read(&delayed_refs->procs_running_refs))
+ wait_event(delayed_refs->wait,
+ atomic_read(&delayed_refs->procs_running_refs)
== 0);
+ btrfs_end_transaction(trans, root);
+ }
+
path = btrfs_alloc_path();
if (!path)
return 0;
--
1.7.7
--
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