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: Tue, 19 Jun 2018 23:00:25 +0000 [thread overview]
Message-ID: <939801BC-1FFD-4770-A491-81E2F3D847C2@mellanox.com> (raw)
In-Reply-To: <20180619114852.lzxwjj7ud5owcyuu@laranjeiro-vm.dev.6wind.com>
> On Jun 19, 2018, at 4:48 AM, Nélio Laranjeiro <nelio.laranjeiro@6wind.com> wrote:
>
> On Mon, Jun 18, 2018 at 05:06:41PM +0000, Yongseok Koh wrote:
>>
>>> 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.
>
> With your modification the function documentation is no more accurate as
> rte_errno is always set.
I still don't agree with that but will send out v2. It's not a big deal.
Thanks,
Yongseok
next prev parent reply other threads:[~2018-06-19 23:00 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
2018-06-19 11:48 ` Nélio Laranjeiro
2018-06-19 23:00 ` Yongseok Koh [this message]
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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