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

From: Florian Fainelli <f.fainelli@gmail.com>
To: Lino Sanfilippo <LinoSanfilippo@gmx.de>,
	Vladimir Oltean <olteanv@gmail.com>
Cc: p.rosenberger@kunbus.com, woojung.huh@microchip.com,
	UNGLinuxDriver@microchip.com, andrew@lunn.ch,
	vivien.didelot@gmail.com, davem@davemloft.net, kuba@kernel.org,
	netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/3] Fix for KSZ DSA switch shutdown
Date: Thu, 9 Sep 2021 09:44:24 -0700	[thread overview]
Message-ID: <2b316d9f-1249-9008-2901-4ab3128eed81@gmail.com> (raw)
In-Reply-To: <8498b0ce-99bb-aef9-05e1-d359f1cad6cf@gmx.de>

On 9/9/2021 9:37 AM, Lino Sanfilippo wrote:
> On 09.09.21 at 17:47, 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".
>>
> 
> Ah, that is interesting.
> 
>> 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?!).
>>
>> 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.
>>
> 
> Right, that was also my conclusion.
> 
>> 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.
> 
> Well actually the SPI shutdown hook gets called which then calls ksz9477_shutdown
> (formerly ksz9477_reset_switch) which then shuts down the switch by
> stopping the worker thread and tearing down the DSA tree (via dsa_tree_shutdown()).
> 
> While it is right that the patch series only fixes the KSZ case for now, the idea was that
> other drivers could use a similar approach in by calling the new function dsa_tree_shutdown()
> in their shutdown handler to make sure that all refs to the master device are released.

It does not scale really well to have individual drivers call 
dsa_tree_shutdown() in their respective .shutdown callback, and in a 
multi-switch configuration, I am not sure what the results would look like.

In premise, each driver ought to be able to call 
dsa_unregister_switch(), along with all of the driver specific shutdown 
and eventually, given proper device ordering the DSA tree would get 
automatically torn down, and then the DSA master's .shutdown() callback 
would be called.

FWIW, the reason why we call .shutdown() in bcmgenet is to turn off DMA 
and clocks, which matters for kexec (DMA) as well as power savings (S5 
mode).
-- 
Florian