linux-kernel - Re: [PATCH 0/3] Fix for KSZ DSA switch shutdown

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Date:   Thu, 9 Sep 2021 09:00:21 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Vladimir Oltean <olteanv@...il.com>,
        Lino Sanfilippo <LinoSanfilippo@....de>,
        Saravana Kannan <saravanak@...gle.com>
Cc:     p.rosenberger@...bus.com, woojung.huh@...rochip.com,
        UNGLinuxDriver@...rochip.com, andrew@...n.ch,
        vivien.didelot@...il.com, davem@...emloft.net, kuba@...nel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] Fix for KSZ DSA switch shutdown

+Saravana,

On 9/9/2021 8:47 AM, Vladimir Oltean wrote:
> On Thu, Sep 09, 2021 at 03:19:52PM +0200, Lino Sanfilippo wrote:
>>> Do you see similar things on your 5.10 kernel?
>>
>> For the master device is see
>>
>> lrwxrwxrwx 1 root root 0 Sep  9 14:10 /sys/class/net/eth0/device/consumer:spi:spi3.0 -> ../../../virtual/devlink/platform:fd580000.ethernet--spi:spi3.0
> 
> So this is the worst of the worst, we have a device link but it doesn't help.
> 
> Where the device link helps is here:
> 
> __device_release_driver
> 	while (device_links_busy(dev))
> 		device_links_unbind_consumers(dev);
> 
> but during dev_shutdown, device_links_unbind_consumers does not get called
> (actually I am not even sure whether it should).
> 
> I've reproduced your issue by making this very simple change:
> 
> diff --git a/drivers/net/ethernet/freescale/enetc/enetc_pf.c b/drivers/net/ethernet/freescale/enetc/enetc_pf.c
> index 60d94e0a07d6..ec00f34cac47 100644
> --- a/drivers/net/ethernet/freescale/enetc/enetc_pf.c
> +++ b/drivers/net/ethernet/freescale/enetc/enetc_pf.c
> @@ -1372,6 +1372,7 @@ static struct pci_driver enetc_pf_driver = {
>   	.id_table = enetc_pf_id_table,
>   	.probe = enetc_pf_probe,
>   	.remove = enetc_pf_remove,
> +	.shutdown = enetc_pf_remove,
>   #ifdef CONFIG_PCI_IOV
>   	.sriov_configure = enetc_sriov_configure,
>   #endif
> 
> on my DSA master driver. This is what the genet driver has "special".
> 
> I was led into grave error by Documentation/driver-api/device_link.rst,
> which I've based my patch on, where it clearly says that device links
> are supposed to help with shutdown ordering (how?!).

I was also under the impression that device links were supposed to help 
with shutdown ordering, because it does matter a lot. One thing that I 
had to work before (and seems like it came back recently) is the 
shutdown ordering between gpio_keys.c and the GPIO controller. If you 
suspend the GPIO controller first, gpio_keys.c never gets a chance to 
keep the GPIO pin configured for a wake-up interrupt, therefore no 
wake-up event happens on key presses, whoops.

> 
> So the question is, why did my DSA trees get torn down on shutdown?
> Basically the short answer is that my SPI controller driver does
> implement .shutdown, and calls the same code path as the .remove code,
> which calls spi_unregister_controller which removes all SPI children..
> 
> When I added this device link, one of the main objectives was to not
> modify all DSA drivers. I was certain based on the documentation that
> device links would help, now I'm not so sure anymore.
> 
> So what happens is that the DSA master attempts to unregister its net
> device on .shutdown, but DSA does not implement .shutdown, so it just
> sits there holding a reference (supposedly via dev_hold, but where from?!)
> to the master, which makes netdev_wait_allrefs to wait and wait.

It's not coming from of_find_net_device_by_node() that's for sure and 
with OF we don't go through the code path calling 
dsa_dev_to_net_device() which does call dev_hold() and then shortly 
thereafter the caller calls dev_put() anyway.

> 
> I need more time for the denial phase to pass, and to understand what
> can actually be done. I will also be away from the keyboard for the next
> few days, so it might take a while. Your patches obviously offer a
> solution only for KSZ switches, we need something more general. If I
> understand your solution, it works not by virtue of there being any
> shutdown ordering guarantee at all, but simply due to the fact that
> DSA's .shutdown hook gets called eventually, and the reference to the
> master gets freed eventually, which unblocks the unregister_netdevice
> call from the master. I don't yet understand why DSA holds a long-term
> reference to the master, that's one thing I need to figure out.
> 

Agreed.
-- 
Florian