[PATCH 5.10 044/103] net: dsa: realtek: register the MDIO bus under devres

From: Greg Kroah-Hartman
Date: Mon Sep 27 2021 - 13:15:08 EST

From: Vladimir Oltean <vladimir.oltean@xxxxxxx>

[ Upstream commit 74b6d7d13307b016f4b5bba8198297824c0ee6df ]

The Linux device model permits both the ->shutdown and ->remove driver
methods to get called during a shutdown procedure. Example: a DSA switch
which sits on an SPI bus, and the SPI bus driver calls this on its
->shutdown method:

-> device_for_each_child(&ctlr->dev, NULL, __unregister);
-> spi_unregister_device(to_spi_device(dev));
-> device_del(&spi->dev);

So this is a simple pattern which can theoretically appear on any bus,
although the only other buses on which I've been able to find it are

-> device_for_each_child(&adap->dev, NULL, __unregister_client);
-> i2c_unregister_device(client);
-> device_unregister(&client->dev);

The implication of this pattern is that devices on these buses can be
unregistered after having been shut down. The drivers for these devices
might choose to return early either from ->remove or ->shutdown if the
other callback has already run once, and they might choose that the
->shutdown method should only perform a subset of the teardown done by
->remove (to avoid unnecessary delays when rebooting).

So in other words, the device driver may choose on ->remove to not
do anything (therefore to not unregister an MDIO bus it has registered
on ->probe), because this ->remove is actually triggered by the
device_shutdown path, and its ->shutdown method has already run and done
the minimally required cleanup.

This used to be fine until the blamed commit, but now, the following
BUG_ON triggers:

void mdiobus_free(struct mii_bus *bus)
/* For compatibility with error handling in drivers. */
if (bus->state == MDIOBUS_ALLOCATED) {

bus->state = MDIOBUS_RELEASED;


In other words, there is an attempt to free an MDIO bus which was not
unregistered. The attempt to free it comes from the devres release
callbacks of the SPI device, which are executed after the device is

I'm not saying that the fact that MDIO buses allocated using devres
would automatically get unregistered wasn't strange. I'm just saying
that the commit didn't care about auditing existing call paths in the
kernel, and now, the following code sequences are potentially buggy:

(a) devm_mdiobus_alloc followed by plain mdiobus_register, for a device
located on a bus that unregisters its children on shutdown. After
the blamed patch, either both the alloc and the register should use
devres, or none should.

(b) devm_mdiobus_alloc followed by plain mdiobus_register, and then no
mdiobus_unregister at all in the remove path. After the blamed
patch, nobody unregisters the MDIO bus anymore, so this is even more
buggy than the previous case which needs a specific bus
configuration to be seen, this one is an unconditional bug.

In this case, the Realtek drivers fall under category (b). To solve it,
we can register the MDIO bus under devres too, which restores the
previous behavior.

Fixes: ac3a68d56651 ("net: phy: don't abuse devres in devm_mdiobus_register()")
Reported-by: Lino Sanfilippo <LinoSanfilippo@xxxxxx>
Reported-by: Alvin Šipraga <alsi@xxxxxxxxxxxxxxx>
Signed-off-by: Vladimir Oltean <vladimir.oltean@xxxxxxx>
Reviewed-by: Andrew Lunn <andrew@xxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
drivers/net/dsa/realtek-smi-core.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/dsa/realtek-smi-core.c b/drivers/net/dsa/realtek-smi-core.c
index 8e49d4f85d48..6bf46d76c028 100644
--- a/drivers/net/dsa/realtek-smi-core.c
+++ b/drivers/net/dsa/realtek-smi-core.c
@@ -368,7 +368,7 @@ int realtek_smi_setup_mdio(struct realtek_smi *smi)
smi->slave_mii_bus->parent = smi->dev;
smi->ds->slave_mii_bus = smi->slave_mii_bus;

- ret = of_mdiobus_register(smi->slave_mii_bus, mdio_np);
+ ret = devm_of_mdiobus_register(smi->dev, smi->slave_mii_bus, mdio_np);
if (ret) {
dev_err(smi->dev, "unable to register MDIO bus %s\n",