From: "Nélio Laranjeiro" <nelio.laranjeiro@6wind.com>
To: Yongseok Koh <yskoh@mellanox.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 13:48:52 +0200 [thread overview]
Message-ID: <20180619114852.lzxwjj7ud5owcyuu@laranjeiro-vm.dev.6wind.com> (raw)
In-Reply-To: <42BB3FF2-80B8-4250-928C-32D509E32DAA@mellanox.com>
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.
--
Nélio Laranjeiro
6WIND
next prev parent reply other threads:[~2018-06-19 11:48 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 [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20180619114852.lzxwjj7ud5owcyuu@laranjeiro-vm.dev.6wind.com \
--to=nelio.laranjeiro@6wind.com \
--cc=adrien.mazarguil@6wind.com \
--cc=dev@dpdk.org \
--cc=shahafs@mellanox.com \
--cc=stable@dpdk.org \
--cc=xuemingl@mellanox.com \
--cc=yskoh@mellanox.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).