Re: [dpdk-dev] [PATCH] net/mlx5: fix error number handling

[Yongseok Koh <yskoh@mellanox.com>]

> On Jun 7, 2018, at 12:39 AM, Nélio Laranjeiro <nelio.laranjeiro@6wind.com> wrote:
> 
> On Wed, Jun 06, 2018 at 11:39:27AM -0700, Yongseok Koh wrote:
>> On Wed, Jun 06, 2018 at 08:55:01AM +0200, Nélio Laranjeiro wrote:
>>> On Tue, Jun 05, 2018 at 09:36:32PM +0000, Yongseok Koh wrote:
>>>>> On Jun 4, 2018, at 11:52 PM, Nélio Laranjeiro <nelio.laranjeiro@6wind.com> wrote:
>>>>> 
>>>>> On Mon, Jun 04, 2018 at 10:37:31AM -0700, Yongseok Koh wrote:
>>>>>> rte_errno should be saved only if error has occurred because rte_errno
>>>>>> could have garbage value.
>>>>>> 
>>>>>> Fixes: a6d83b6a9209 ("net/mlx5: standardize on negative errno values")
>>>>>> Cc: stable@dpdk.org
>>>>>> 
>>>>>> Signed-off-by: Yongseok Koh <yskoh@mellanox.com>
>>>>>> ---
>>>>>> drivers/net/mlx5/mlx5_flow.c | 3 ++-
>>>>>> 1 file changed, 2 insertions(+), 1 deletion(-)
>>>>>> 
>>>>>> diff --git a/drivers/net/mlx5/mlx5_flow.c b/drivers/net/mlx5/mlx5_flow.c
>>>>>> index 994be05be..eaffe7495 100644
>>>>>> --- a/drivers/net/mlx5/mlx5_flow.c
>>>>>> +++ b/drivers/net/mlx5/mlx5_flow.c
>>>>>> @@ -3561,7 +3561,8 @@ mlx5_fdir_filter_delete(struct rte_eth_dev *dev,
>>>>>> 		/* The flow does not match. */
>>>>>> 		continue;
>>>>>> 	}
>>>>>> -	ret = rte_errno; /* Save rte_errno before cleanup. */
>>>>>> +	if (ret)
>>>>>> +		ret = rte_errno; /* Save rte_errno before cleanup. */
>>>>>> 	if (flow)
>>>>>> 		mlx5_flow_list_destroy(dev, &priv->flows, flow);
>>>>>> exit:
>>>>>> -- 
>>>>>> 2.11.0
>>>>> 
>>>>> This patch is not enough, the returned value being -rte_errno if no
>>>>> error is detected by the function it cannot set rte_errno nor return it.
>>>> 
>>>> We may need to refactor this kind of code (saving and restoring rte_errno). I
>>>> still don't understand why we should preserve rte_errno like this.
>>>> 
>>>> Even if this function returns success, there's no obligation to preserve
>>>> rte_errno in the function. Once it is called, the ownership of rte_errno belongs
>>>> to this function.
>>>> 
>>>> I can't find how we define this per-lcore variable but, from
>>>> the man page of errno,
>>>> 
>>>>       The  <errno.h>  header  file  defines  the integer variable errno, which
>>>>       is set by system calls and some library functions in the event of an
>>>>       error to indicate what went wrong.  Its value is significant only when
>>>>       the return value of the call indicated an error (i.e., -1 from most
>>>>       system calls; -1 or NULL from most library  functions);
>>>>       a function that succeeds is allowed to change errno.
>>>> 
>>>> So, I still think an API can change rte_errno even if it succeeds, no need to
>>>> preserve it. If needed, the caller has to save it.
>>> 
>>> Functions in this PMD are defined as is:
>>> 
>>>  * @return
>>>  *   0 on success, a negative errno value otherwise and rte_errno is set.
>>> 
>>> Which means rte_errno is only modified in case of error.
>>> 
>>> This fix does not respect the documentation of the function or any other
>>> function of the PMD which can return errors.
>> 
>> That's logically a wrong interpretation. According to the description, if
>> returning error, rte_errno is set but the opposite isn't always true. Even if
>> rte_errno is set, it doesn't mean there's an error. So the description coincides
>> with that of errno. If you want to enforce preserving rte_errno in case of
>> success, you should amend the documentation.
>> 
>>> rte_errno is only set if an error is encountered and contains only the error
>>> code of the first error sub-sequent ones are considered consequences of the
>>> first one and thus not preserved.
>>> 
>>> Not preserving the rte_errno in roll backs is equivalent to not setting
>>> it at all as a function called by the rollback may also set it, example:
>>> 
>>> {
>>>    void * a;
>>> 
>>>    foo_do();
>>>    a  = malloc(10);
>>>    if (!a) {
>>>    	rte_errno = ENOMEM;
>>> 	foo_undo();
>> 
>> This example is weird. You can simply set rte_errno after foo_undo() in this
>> case.
>> 
>>> 	return -rte_errno;
>>>    }
>>> }
>>> 
>>> If foo_undo() also encounter an error it will modify the rte_errno which
>>> may have a value different from ENOMEM, for the callee won't be informed
>>> the error is due to a memory issue and thus cannot make counter parts.
>>> In such situation the rte_errno must be preserved to keep the ENOMEM
>>> error code.
>> 
>> I knew it. That's why rte_errno is saved before calling another API which may
>> change the rte_errno inside. But, we are talking about a case where an API
>> returns success. If caller is supposed to save rte_errno (when it's needed), why
>> does callee have to put some effort to preserve it even in case of success? If
>> rte_errno must be preserved even in case of success, we have to make a big
>> change to preserve rte_errno for cases where a void function is called (or cases
>> where we don't check its return value of non-void function).
>> 
>>> This is also the main reason almost all system function only update
>>> errno when no error is encountered.
>> 
>> 'Almost' doesn't mean 'all", does it? It is true that such functions must update
>> errno when it returns error but it doesn't care about the value when it returns
>> success. Like the man page I attached above, the errno is significant only when
>> it returns an error. And "a function that succeeds is allowed to change errno."
> 
> It is "almost" because a system function touching the errno when the
> function succeed it not common.  But as the man page says it is not
> impossible.
> 
>> So, the decision point is whether we want to preserve rte_errno in case of
>> success? My opinion is no.
> 
> I did not understood it was only a concern about the success of the
> function, even it is better to avoid as most as possible a useless
> store, in this specific case, as errno (rte_errno) has a garbage value,
> I fully agree with you.

Nelio,

Do you still want me to make any change for this patch?
Let me know if any.

Thanks,
Yongseok