Re: [RFC net] net: dsa: Add missing reference counting

On Thu, 7 May 2020 at 01:45, Florian Fainelli [off-list ref] wrote:

On 5/6/2020 2:40 PM, Vladimir Oltean wrote:

quoted

Hi Florian,

On Thu, 7 May 2020 at 00:24, Florian Fainelli [off-list ref] wrote:

quoted

On 5/5/2020 2:23 PM, Vivien Didelot wrote:

quoted

On Tue,  5 May 2020 14:02:53 -0700, Florian Fainelli [off-list ref] wrote:

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If we are probed through platform_data we would be intentionally
dropping the reference count on master after dev_to_net_device()
incremented it. If we are probed through Device Tree,
of_find_net_device() does not do a dev_hold() at all.

Ensure that the DSA master device is properly reference counted by
holding it as soon as the CPU port is successfully initialized and later
released during dsa_switch_release_ports(). dsa_get_tag_protocol() does
a short de-reference, so we hold and release the master at that time,
too.

Fixes: 83c0afaec7b7 ("net: dsa: Add new binding implementation")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

Reviewed-by: Vivien Didelot <redacted>

Andrew, Vladimir, any thoughts on that?
--
Florian

I might be completely off because I guess I just don't understand what
is the goal of keeping a reference to the DSA master in this way for
the entire lifetime of the DSA switch. I think that dev_hold is for
short-term things that cannot complete atomically, but I think that
you are trying to prevent the DSA master from getting freed from under
our feet, which at the moment would fault the kernel instantaneously?

Yes, that's the idea, you should not be able to rmmod/unbind the DSA
master while there is a DSA switch tree hanging off of it.

quoted

If this is correct, it certainly doesn't do what it intends to do:
echo 0000\:00\:00.5> /sys/bus/pci/drivers/mscc_felix/unbind
[   71.576333] unregister_netdevice: waiting for swp0 to become free.
Usage count = 1
(hangs there)

Is this with the sja1105 switch hanging off felix?

Yes, but it actually doesn't matter that the DSA master is a DSA slave too.

If so, is not it
working as expected because you still have sja1150 being bound to one of
those ports? If not, then I will look into why.

I just unbound the driver for the DSA master and the shell got stuck
in kernel process context telling me that it's waiting for the
reference to be freed. So I think it's just that my "expected" is not
the same as yours - it looks like what I'm doing would qualify as
"incorrect usage".

quoted

But if I'm right and that's indeed what you want to achieve, shouldn't
we be using device links instead?
https://www.kernel.org/doc/html/v4.14/driver-api/device_link.html

device links could work but given that the struct device and struct
net_device have almost the same lifetime, with the net_device being a
little bit shorter, and that is what DSA uses, I am not sure whether
device link would bring something better.

At the very least, I think they would bring us graceful teardown of
consumers of the DSA master device.

--
Florian

Thanks,
-Vladimir