On Sun, Dec 08, 2013 at 12:26:29AM +0000, Filipe David Borba Manana wrote:
> Currently extent-tree.c:btrfs_lookup_extent_info() can miss the lookup
> of skinny extent items. This can happen when the execution flow is the
> following:
>
> * We do an extent tree lookup and fail to find a skinny extent item;
>
> * As a result, we attempt to see if a non-skinny extent item exists,
> either by looking at previous item in the leaf or by doing another
> full extent tree search;
>
> * We have a transaction and then we check for a matching delayed ref
> head in the transaction's delayed refs rbtree;
>
> * We find such delayed ref head and then we try to lock it with a
> call to mutex_trylock();
>
> * The lock was contended so we jump to the label "again", which repeats
> the extent tree search but for a non-skinny extent item, because we set
> previously metadata variable to 0 and the search key to look for a
> non-skinny extent-item;
>
> * After the jump (and after releasing the transaction's delayed refs
> lock), a skinny extent item might have been added to the extent tree
> but we will miss it because metadata is set to 0 and the search key
> is set for a non-skinny extent-item.
>
> The fix here is to not reset metadata to 0 and to jump to the initial search
> key setup if the delayed ref head is contended, instead of jumping directly
> to the extent tree search label ("again").
>
> This issue was found while investigating the issue reported at Bugzilla 64961.
>
> David Sterba suspected this function was missing extent items, and that
> this could be caused by the last change to this function, which was made
> in the following patch:
>
> [PATCH] Btrfs: optimize btrfs_lookup_extent_info()
> (commit 74be9510876a66ad9826613ac8a526d26f9e7f01)
>
> But in fact this issue already existed before, because after failing to find
> a skinny extent item, the code set the search key for a non-skinny extent
> item, and on contention of a matching delayed ref head it would not search
> the extent tree for a skinny extent item anymore.
Make sense.
Reviewed-by: Liu Bo <[email protected]>
-liubo
>
> Signed-off-by: Filipe David Borba Manana <[email protected]>
> ---
> fs/btrfs/extent-tree.c | 22 ++++++++++------------
> 1 file changed, 10 insertions(+), 12 deletions(-)
>
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 62d433f..c9f020e 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -769,20 +769,19 @@ int btrfs_lookup_extent_info(struct btrfs_trans_handle
> *trans,
> if (!path)
> return -ENOMEM;
>
> - if (metadata) {
> - key.objectid = bytenr;
> - key.type = BTRFS_METADATA_ITEM_KEY;
> - key.offset = offset;
> - } else {
> - key.objectid = bytenr;
> - key.type = BTRFS_EXTENT_ITEM_KEY;
> - key.offset = offset;
> - }
> -
> if (!trans) {
> path->skip_locking = 1;
> path->search_commit_root = 1;
> }
> +
> +search_again:
> + key.objectid = bytenr;
> + key.offset = offset;
> + if (metadata)
> + key.type = BTRFS_METADATA_ITEM_KEY;
> + else
> + key.type = BTRFS_EXTENT_ITEM_KEY;
> +
> again:
> ret = btrfs_search_slot(trans, root->fs_info->extent_root,
> &key, path, 0, 0);
> @@ -790,7 +789,6 @@ again:
> goto out_free;
>
> if (ret > 0 && metadata && key.type == BTRFS_METADATA_ITEM_KEY) {
> - metadata = 0;
> if (path->slots[0]) {
> path->slots[0]--;
> btrfs_item_key_to_cpu(path->nodes[0], &key,
> @@ -857,7 +855,7 @@ again:
> mutex_lock(&head->mutex);
> mutex_unlock(&head->mutex);
> btrfs_put_delayed_ref(&head->node);
> - goto again;
> + goto search_again;
> }
> if (head->extent_op && head->extent_op->update_flags)
> extent_flags |= head->extent_op->flags_to_set;
> --
> 1.7.9.5
>
> --
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