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

[Nélio Laranjeiro]

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 at 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 at 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 at dpdk.org
> >>>>>> 
> >>>>>> Signed-off-by: Yongseok Koh <yskoh at 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