Re: [dpdk-dev] [PATCH] net/mlx5: handle expected errno properly

[Yongseok Koh <yskoh@mellanox.com>]

> On Aug 28, 2018, at 8:52 PM, Jack Min <jackmin@mellanox.com> wrote:
> 
> On Tue, Aug 28, 2018 at 01:42:18PM -0700, Yongseok Koh wrote:
>> On Fri, Aug 24, 2018 at 02:45:00PM +0800, Jack MIN wrote:
>>> On Thu, Aug 23, 2018 at 02:08:09PM -0700, Yongseok Koh wrote:
>>>> On Thu, Aug 23, 2018 at 02:38:51PM +0800, Xiaoyu Min wrote:
>>>>> rte_errno is a per thread variable and is widely used as an
>>>>> error indicator, which means a function could affect
>>>>> other functions' results by setting rte_errno carelessly
>>>>> 
>>>>> During rxq setup, an EINVAL rte_errno is expected since
>>>>> the queues are not created yet
>>>>> So rte_errno is cleared when it is EINVAL as expected
>>>>> 
>>>>> Signed-off-by: Xiaoyu Min <jackmin@mellanox.com>
>>>>> ---
>>>>> drivers/net/mlx5/mlx5_rxq.c | 20 +++++++++++++++-----
>>>>> 1 file changed, 15 insertions(+), 5 deletions(-)
>>>>> 
>>>>> diff --git a/drivers/net/mlx5/mlx5_rxq.c b/drivers/net/mlx5/mlx5_rxq.c
>>>>> index 1f7bfd4..e7056e8 100644
>>>>> --- a/drivers/net/mlx5/mlx5_rxq.c
>>>>> +++ b/drivers/net/mlx5/mlx5_rxq.c
>>>>> @@ -443,6 +443,7 @@
>>>>> 	struct mlx5_rxq_data *rxq = (*priv->rxqs)[idx];
>>>>> 	struct mlx5_rxq_ctrl *rxq_ctrl =
>>>>> 		container_of(rxq, struct mlx5_rxq_ctrl, rxq);
>>>>> +	int ret = 0;
>>>>> 
>>>>> 	if (!rte_is_power_of_2(desc)) {
>>>>> 		desc = 1 << log2above(desc);
>>>>> @@ -459,13 +460,21 @@
>>>>> 		rte_errno = EOVERFLOW;
>>>>> 		return -rte_errno;
>>>>> 	}
>>>>> -	if (!mlx5_rxq_releasable(dev, idx)) {
>>>>> +	ret = mlx5_rxq_releasable(dev, idx);
>>>>> +	if (!ret) {
>>>>> 		DRV_LOG(ERR, "port %u unable to release queue index %u",
>>>>> 			dev->data->port_id, idx);
>>>>> 		rte_errno = EBUSY;
>>>>> 		return -rte_errno;
>>>>> +	} else if (ret == -EINVAL) {
>>>>> +		/**
>>>>> +		 * on the first time, rx queue doesn't exist,
>>>>> +		 * so just ignore this error and reset rte_errno.
>>>>> +		 */
>>>>> +		rte_errno = 0;
>>>> 
>>>> Unless this function returns failure, the rte_errno will be ignored by caller
>>>> and caller shouldn't assume rte_errno has 0. Caller will assume it is garbage
>>>> data in case of success. So we can silently ignore this case. Does it cause any
>>>> issue in application side?
>>>> 
>>> Not application side but mlx5 PMD this time:
>>> **mlx5_fdir_filter_delete** 
>>> which just _return -rte_errno;_
>> 
>> Looks like an error. mlx5_fdir_filter_delete() can't be like that. We seem to
>> have lost the code while refactoring it. Let take it offline.
>> 
> Sure ~
> 
>>> For sure, _mlx5_fdir_filter_delete_ should be more defensive, should not assume
>>> rte_errno is zero if no error happened.
>>> However if the caller know that an error will happen and rte_errno will become
>>> meaningless (garbage) for the succeeding functions, Catching the expected error
>>> and resetting rte_errno will be better. What do you think?
>> 
>> Still don't understand clearly. There would be many other similar cases where we
>> don't clear rte_errno when returning success. I don't understand why this case
>> should be taken as a special one??
>> 
> No, the _mlx5_rxq_releasable()_ returns *error* (-EINVAL)(the rxq doesn't exist)
> as my understanding
> We only check if rxq is not releasable(==0) in previouse version but we didn't
> check if function returns error or success

I think that was intended. The case where rxq doesn't exist is silently ignored.
I know it is a little weird but still I don't see any need to clear rte_errno.


Thanks,
Yongseok