Re: [PATCH v3 6/6] net/failsafe: fix removed device handling

[Matan Azrad <matan@mellanox.com>]

Hi Gaetan

From: Gaëtan Rivet, Monday, January 8, 2018 12:58 PM
> Hi Matan,
> 
> Sorry for the delay on this.
> 

It's OK in spite of I need to fetch it back :)

> On Wed, Dec 20, 2017 at 10:58:29AM +0000, Matan Azrad wrote:
> > Hi Gaetan
> >
> > > -----Original Message-----
> > > From: Gaëtan Rivet [mailto:gaetan.rivet@6wind.com]
> > > Sent: Wednesday, December 20, 2017 12:22 AM
> > > To: Matan Azrad <matan@mellanox.com>
> > > Cc: Adrien Mazarguil <adrien.mazarguil@6wind.com>; Thomas Monjalon
> > > <thomas@monjalon.net>; dev@dpdk.org
> > > Subject: Re: [PATCH v3 6/6] net/failsafe: fix removed device
> > > handling
> > >
> > > Hi Matan,
> > >
> > > On Tue, Dec 19, 2017 at 05:10:15PM +0000, Matan Azrad wrote:
> > > > There is time between the physical removal of the device until
> > > > sub-device PMDs get a RMV interrupt. At this time DPDK PMDs and
> > > > applications still don't know about the removal and may call
> > > > sub-device control operation which should return an error.
> > > >
> > > > In previous code this error is reported to the application
> > > > contrary to fail-safe principle that the app should not be aware of
> device removal.
> > > >
> > > > Add an removal check in each relevant control command error flow
> > > > and prevent an error report to application when the sub-device is
> removed.
> > > >
> > > > Fixes: a46f8d5 ("net/failsafe: add fail-safe PMD")
> > > > Fixes: b737a1e ("net/failsafe: support flow API")
> 
> As stated previously, please do not include those fixes lines.
> 
> > > >
> > > > Signed-off-by: Matan Azrad <matan@mellanox.com>
> > > > ---
> > >
> > > <snip>
> > >
> > > > +/*
> > > > + * Check if error should be reported to the user.
> > > > + */
> > > > +static inline bool
> > > > +fs_is_error(struct sub_device *sdev, int err) {
> > > > +	/* A device removal shouldn't be reported as an error. */
> > > > +	if (err == 0 || sdev->remove == 1 || err == -EIO)
> > > > +		return false;
> > > > +	return true;
> > > > +}
> > >
> > > This is better, thanks.
> > >
> > > However is there a reason you did not follow the same pattern as
> > > ethdev with eth_err? I see the two functions as similar in their
> > > intent, making them close to each other would be clearer to a reader
> > > being familiar with the ethdev API and that would be interested in fail-
> safe.
> > >
> > > What do you think?
> > >
> >
> > I think that there is a real different between eth_err function to
> fs_is_error:
> > ethdev uses eth_err function to adjust removal return value to be -EIO.
> > fail-safe uses fs_is_error function to check if an error should be reported to
> the user to save the fail-safe principle that the app should not be aware of
> device removal  -  this is the main idea that also causes me to change the
> name from fs_is_removed to fs_is_error.
> 
> I would have preferred if it followed the same pattern as ethdev (that
> function be used to adjust the return value, not performing a flag check).
> 
> While better on its own, the pattern:
> 
>     if (fs_is_error(sdev, err)) {
>             ERROR("xxxx");
>             return err;
>     }
> 
> is dangerous, as then the author is forbidden from returning err, assuming
> err could be -EIO. He or she would be forced to return an explicit "0".
> To be clear, here would be an easy mistake to do:
> 
>     if (fs_is_error(sdev, err)) {
>             ERROR("xxxx");
>     }
>     return err;
> 
> And this kind of code-flow is not unusual, or even unwanted.
> I dislike having this kind of implicit rule derived from using a helper such as
> fs_is_error().
> 
> The alternative
> 
>     if ((err = fs_err(sdev, err))) {
>             ERROR("xxxx");
>             return err;
>     }
> 
> Forces the value err to be set to the correct one.
> 
Good point, will change it.

> This mistake can already be found in your patch:
> 
> > @@ -150,7 +150,7 @@
> >                         continue;
> >                 local_ret = rte_flow_destroy(PORT_ID(sdev),
> >                                 flow->flows[i], error);
> > -               if (local_ret) {
> > +               if (fs_is_error(sdev, local_ret)) {
> >                         ERROR("Failed to destroy flow on sub_device %d: %d",
> >                                         i, local_ret);
> >                         if (ret == 0)
> 

Sorry, I can't see any issue here.

> Your environment does not include the function, but this is within
> fs_flow_destroy (please update to include the context by the way it helps a
> lot the review :). Afterward, line 162 ret is directly used as return value.
> 
I don't understand what do you mean.

> Also, fs_err() would need to transform rte_errno when relevant (mostly in
> failsafe_flow.c I think).
> 
Your suggestion is always to update rte_errno to 0 in case the error is because of removal?

> This is the kind of subtlety that needs to be avoided when designing APIs,
> even internal ones. This will induce errors afterward and complicate the
> maintenance of the codebase.
>

Thanks for the lesson! 
 
> Best regards,
> 
> --
> Gaëtan Rivet
> 6WIND