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

["Nélio Laranjeiro" <nelio.laranjeiro@6wind.com>]

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.  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();
	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.
This is also the main reason almost all system function only update
errno when no error is encountered.

Regards,

-- 
Nélio Laranjeiro
6WIND