From: Yongseok Koh <yskoh@mellanox.com>
To: "Nélio Laranjeiro" <nelio.laranjeiro@6wind.com>
Cc: Adrien Mazarguil <adrien.mazarguil@6wind.com>,
"dev@dpdk.org" <dev@dpdk.org>,
Shahaf Shuler <shahafs@mellanox.com>,
dpdk stable <stable@dpdk.org>,
"Xueming(Steven) Li" <xuemingl@mellanox.com>
Subject: Re: [dpdk-stable] [PATCH] net/mlx5: fix error number handling
Date: Mon, 18 Jun 2018 17:06:41 +0000 [thread overview]
Message-ID: <42BB3FF2-80B8-4250-928C-32D509E32DAA@mellanox.com> (raw)
In-Reply-To: <20180607073944.zewdysx6ddrdygoz@laranjeiro-vm.dev.6wind.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
next prev parent reply other threads:[~2018-06-18 17:06 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-06-04 17:37 Yongseok Koh
2018-06-05 6:52 ` Nélio Laranjeiro
2018-06-05 21:36 ` Yongseok Koh
2018-06-06 6:55 ` Nélio Laranjeiro
2018-06-06 18:39 ` Yongseok Koh
2018-06-07 7:39 ` Nélio Laranjeiro
2018-06-18 17:06 ` Yongseok Koh [this message]
2018-06-19 11:48 ` Nélio Laranjeiro
2018-06-19 23:00 ` Yongseok Koh
2018-06-20 7:05 ` Nélio Laranjeiro
2018-06-19 23:13 ` [dpdk-stable] [PATCH v2] " Yongseok Koh
2018-06-20 7:02 ` Nélio Laranjeiro
2018-06-21 10:57 ` Shahaf Shuler
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