On 07/15/2011 01:26 AM, Josef Bacik wrote:
> Hit this nice little deadlock. What happens is this
>
> __btrfs_end_transaction with throttle set, --use_count so it equals 0
> btrfs_commit_transaction
> <somebody else actually manages to start the commit>
> btrfs_end_transaction --use_count so now its -1 <== BAD
> we just return and wait on the transaction
>
> This is bad because we just return after our use_count is -1 and don't let go
> of our num_writer count on the transaction, so the guy committing the
> transaction just sits there forever. Fix this by inc'ing our use_count if we're
> going to call commit_transaction so that if we call btrfs_end_transaction it's
> valid. Thanks,
>
> Signed-off-by: Josef Bacik <josef@xxxxxxxxxx>
> ---
> fs/btrfs/transaction.c | 13 ++++++++++---
> 1 files changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
> index 654755b..00b81fb5 100644
> --- a/fs/btrfs/transaction.c
> +++ b/fs/btrfs/transaction.c
> @@ -497,10 +497,17 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
> }
>
> if (lock && cur_trans->blocked && !cur_trans->in_commit) {
> - if (throttle)
> + if (throttle) {
> + /*
> + * We may race with somebody else here so end up having
> + * to call end_transaction on ourselves again, so inc
> + * our use_count.
> + */
> + trans->use_count++;
> return btrfs_commit_transaction(trans, root);
> - else
> + } else {
> wake_up_process(info->transaction_kthread);
> + }
> }
>
> WARN_ON(cur_trans != info->running_transaction);
> @@ -1225,7 +1232,7 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans,
> if (cur_trans->in_commit) {
> spin_unlock(&cur_trans->commit_lock);
> atomic_inc(&cur_trans->use_count);
> - btrfs_end_transaction(trans, root);
> + __btrfs_end_transaction(trans, root, 0, 1);
>
Looks good.
BTW, btrfs_end_transaction(trans, root) is just __btrfs_end_transaction(trans, root, 0, 1).
thanks,
liubo
> ret = wait_for_commit(root, cur_trans);
> BUG_ON(ret);
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