On Thu, Jul 04, 2013 at 06:29:23PM +0300, Alex Lyakas wrote:
> > @@ -7363,6 +7365,12 @@ int btrfs_drop_snapshot(struct btrfs_root *root,
> > wc->reada_count = BTRFS_NODEPTRS_PER_BLOCK(root);
> >
> > while (1) {
> > + if (!for_reloc && btrfs_fs_closing(root->fs_info)) {
> > + pr_debug("btrfs: drop snapshot early exit\n");
> > + err = -EAGAIN;
> > + goto out_end_trans;
> > + }
> Here you exit the loop, but the "drop_progress" in the root item is
> incorrect. When the system is remounted, and snapshot deletion
> resumes, it seems that it tries to resume from the EXTENT_ITEM that
> does not exist anymore, and [1] shows that btrfs_lookup_extent_info()
> simply does not find the needed extent.
> So then I hit panic in walk_down_tree():
> BUG: wc->refs[level - 1] == 0
>
> I fixed it like follows:
> There is a place where btrfs_drop_snapshot() checks if it needs to
> detach from transaction and re-attach. So I moved the exit point there
> and the code is like this:
>
> if (btrfs_should_end_transaction(trans, tree_root) ||
> (!for_reloc && btrfs_need_cleaner_sleep(root))) {
> ret = btrfs_update_root(trans, tree_root,
> &root->root_key,
> root_item);
> if (ret) {
> btrfs_abort_transaction(trans, tree_root, ret);
> err = ret;
> goto out_end_trans;
> }
>
> btrfs_end_transaction_throttle(trans, tree_root);
> if (!for_reloc && btrfs_need_cleaner_sleep(root)) {
> err = -EAGAIN;
> goto out_free;
> }
> trans = btrfs_start_transaction(tree_root, 0);
> ...
>
> With this fix, I do not hit the panic, and snapshot deletion proceeds
> and completes alright after mount.
>
> Do you agree to my analysis or I am missing something? It seems that
> Josef's btrfs-next still has this issue (as does Chris's for-linus).
Sound analysis and I agree with the fix. The clean-by-one patch has been
merged into 3.10 so we need a stable fix for that.
thanks,
david
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