netdev - Re: [PATCH net] net/tls: Act on going down event

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Date:   Thu, 13 Feb 2020 22:07:19 +0200
From:   Or Gerlitz <gerlitz.or@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Or Gerlitz <ogerlitz@...lanox.com>,
        Tariq Toukan <tariqt@...lanox.com>,
        Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: [PATCH net] net/tls: Act on going down event

On Thu, Feb 13, 2020 at 8:34 PM Jakub Kicinski <kuba@...nel.org> wrote:
> On Thu, 13 Feb 2020 16:54:07 +0000 Or Gerlitz wrote:
> > By the time of the down event, the netdevice stop ndo was
> > already called and the nic driver is likely to destroy the HW
> > objects/constructs which are used for the tls_dev_resync op.

>> @@ -1246,7 +1246,7 @@ static int tls_dev_event(struct notifier_block *this, unsigned long event,
> > -     case NETDEV_DOWN:
> > +     case NETDEV_GOING_DOWN:

> Now we'll have two race conditions. 1. Traffic is still running while
> we remove the connection state. 2. We clean out the state and then
> close the device. Between the two new connections may be installed.
>
> I think it's an inherently racy to only be able to perform clean up
> while the device is still up.

good points, I have to think on both of them and re/sync (..) with
the actual design details here, I came across this while working
on something else which is still more of a research so just throwing
a patch here was moving too fast.

Repeating myself -- by the time of the down event, the netdevice
stop ndo was already called and the nic driver is likely to destroy
HW objects/constructs which are used for the tls_dev_resync op.

For example suppose a NIC driver uses some QP/ring to post resync
request to the HW and these rings are part of the driver's channels and
the channels are destroyed when the stop ndo is called - the tls code
here uses RW synchronization between invoking the resync driver op
to the down event handling. But the down event comes only after these
resources were destroyed, too late. If we will safely/stop the offload
at the going down stage, we can avoid a much more complex and per nic
driver locking which I guess would be needed to protect against that race.

thoughts?

- so here's
the point -- the driver resync op may use some HW

>
> >               return tls_device_down(dev);
> >       }
> >       return NOTIFY_DONE;