On Wed, Aug 12, 2015 at 03:44:51PM +0100, fdmanana@xxxxxxxxxx wrote:
> From: Filipe Manana <fdmanana@xxxxxxxx>
>
> While we are committing a transaction, it's possible the previous one is
> still finishing its commit and therefore we wait for it to finish first.
> However we were not checking if that previous transaction ended up getting
> aborted after we waited for it to commit, so we ended up committing the
> current transaction which can lead to fs corruption because the new
> superblock can point to trees that have had one or more nodes/leafs that
> were never durably persisted.
> The following sequence diagram exemplifies how this is possible:
>
> CPU 0 CPU 1
>
> transaction N starts
>
> (...)
>
> btrfs_commit_transaction(N)
>
> cur_trans->state = TRANS_STATE_COMMIT_START;
> (...)
> cur_trans->state = TRANS_STATE_COMMIT_DOING;
> (...)
>
> cur_trans->state = TRANS_STATE_UNBLOCKED;
> root->fs_info->running_transaction = NULL;
>
> btrfs_start_transaction()
> --> starts transaction N + 1
>
> btrfs_write_and_wait_transaction(trans, root);
> --> starts writing all new or COWed ebs created
> at transaction N
>
> creates some new ebs, COWs some
> existing ebs but doesn't COW or
> deletes eb X
>
> btrfs_commit_transaction(N + 1)
> (...)
> cur_trans->state = TRANS_STATE_COMMIT_START;
> (...)
> wait_for_commit(root, prev_trans);
> --> prev_trans == transaction N
>
> btrfs_write_and_wait_transaction() continues
> writing ebs
> --> fails writing eb X, we abort transaction N
> and set bit BTRFS_FS_STATE_ERROR on
> fs_info->fs_state, so no new transactions
> can start after setting that bit
>
> cleanup_transaction()
> btrfs_cleanup_one_transaction()
> wakes up task at CPU 1
>
> continues, doesn't abort because
> cur_trans->aborted (transaction N + 1)
> is zero, and no checks for bit
> BTRFS_FS_STATE_ERROR in fs_info->fs_state
> are made
>
> btrfs_write_and_wait_transaction(trans, root);
> --> succeeds, no errors during writeback
>
> write_ctree_super(trans, root, 0);
> --> succeeds
> --> we have now a superblock that points us
> to some root that uses eb X, which was
> never written to disk
>
> In this scenario future attempts to read eb X from disk results in an
> error message like "parent transid verify failed on X wanted Y found Z".
>
> So fix this by aborting the current transaction if after waiting for the
> previous transaction we verify that it was aborted.
Looks good to me.
Reviewed-by: Liu Bo <bo.li.liu@xxxxxxxxxx>
Thanks,
-liubo
>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Filipe Manana <fdmanana@xxxxxxxx>
> Reviewed-by: Josef Bacik <jbacik@xxxxxx>
> ---
>
> V2: Added missing cc to stable and included Josef's reviewed-by tag.
> No code changes.
>
> fs/btrfs/transaction.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
> index 51e0f0d..d15a43f 100644
> --- a/fs/btrfs/transaction.c
> +++ b/fs/btrfs/transaction.c
> @@ -1893,8 +1893,11 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans,
> spin_unlock(&root->fs_info->trans_lock);
>
> wait_for_commit(root, prev_trans);
> + ret = prev_trans->aborted;
>
> btrfs_put_transaction(prev_trans);
> + if (ret)
> + goto cleanup_transaction;
> } else {
> spin_unlock(&root->fs_info->trans_lock);
> }
> --
> 2.1.3
>
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