Re: [PATCH net 1/2] net: dsa: don't allocate the slave_mii_bus using devres

[Lino Sanfilippo <LinoSanfilippo@xxxxxx>]

Hi Vladimir,

On 20.09.21 at 23:42, Vladimir Oltean wrote:
> 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:
>
> spi_unregister_controller
> -> 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
> I2C:
>
> i2c_del_adapter
> -> 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) {
> 		kfree(bus);
> 		return;
> 	}
>
> 	BUG_ON(bus->state != MDIOBUS_UNREGISTERED);
> 	bus->state = MDIOBUS_RELEASED;
>
> 	put_device(&bus->dev);
> }
>
> 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
> unregistered.
>
> 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, DSA falls into category (a), it tries to be helpful and
> registers an MDIO bus on behalf of the switch, which might be on such a
> bus. I've no idea why it does it under devres.
>
> It does this on probe:
>
> 	if (!ds->slave_mii_bus && ds->ops->phy_read)
> 		alloc and register mdio bus
>
> and this on remove:
>
> 	if (ds->slave_mii_bus && ds->ops->phy_read)
> 		unregister mdio bus
>
> I _could_ imagine using devres because the condition used on remove is
> different than the condition used on probe. So strictly speaking, DSA
> cannot determine whether the ds->slave_mii_bus it sees on remove is the
> ds->slave_mii_bus that _it_ has allocated on probe. Using devres would
> have solved that problem. But nonetheless, the existing code already
> proceeds to unregister the MDIO bus, even though it might be
> unregistering an MDIO bus it has never registered. So I can only guess
> that no driver that implements ds->ops->phy_read also allocates and
> registers ds->slave_mii_bus itself.
>
> So in that case, if unregistering is fine, freeing must be fine too.
>
> Stop using devres and free the MDIO bus manually. This will make devres
> stop attempting to free a still registered MDIO bus on ->shutdown.
>
> Fixes: ac3a68d56651 ("net: phy: don't abuse devres in devm_mdiobus_register()")
> Reported-by: Lino Sanfilippo <LinoSanfilippo@xxxxxx>
> Signed-off-by: Vladimir Oltean <vladimir.oltean@xxxxxxx>
> ---
>  net/dsa/dsa2.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/net/dsa/dsa2.c b/net/dsa/dsa2.c
> index f14897d9b31d..274018e9171c 100644
> --- a/net/dsa/dsa2.c
> +++ b/net/dsa/dsa2.c
> @@ -880,7 +880,7 @@ static int dsa_switch_setup(struct dsa_switch *ds)
>  	devlink_params_publish(ds->devlink);
>
>  	if (!ds->slave_mii_bus && ds->ops->phy_read) {
> -		ds->slave_mii_bus = devm_mdiobus_alloc(ds->dev);
> +		ds->slave_mii_bus = mdiobus_alloc();
>  		if (!ds->slave_mii_bus) {
>  			err = -ENOMEM;
>  			goto teardown;
> @@ -890,13 +890,16 @@ static int dsa_switch_setup(struct dsa_switch *ds)
>
>  		err = mdiobus_register(ds->slave_mii_bus);
>  		if (err < 0)
> -			goto teardown;
> +			goto free_slave_mii_bus;
>  	}
>
>  	ds->setup = true;
>
>  	return 0;
>
> +free_slave_mii_bus:
> +	if (ds->slave_mii_bus && ds->ops->phy_read)
> +		mdiobus_free(ds->slave_mii_bus);
>  teardown:
>  	if (ds->ops->teardown)
>  		ds->ops->teardown(ds);
> @@ -921,8 +924,11 @@ static void dsa_switch_teardown(struct dsa_switch *ds)
>  	if (!ds->setup)
>  		return;
>
> -	if (ds->slave_mii_bus && ds->ops->phy_read)
> +	if (ds->slave_mii_bus && ds->ops->phy_read) {
>  		mdiobus_unregister(ds->slave_mii_bus);
> +		mdiobus_free(ds->slave_mii_bus);
> +		ds->slave_mii_bus = NULL;
> +	}
>
>  	dsa_switch_unregister_notifier(ds);
>
>

I applied this patch on top of your series "Make DSA switch drivers compatible with
masters which unregister on shutdown" and now the shutdown works as expected (i.e.
no hang as without your patches and no kernel BUG as with only the above mentioned
series applied).

Great job, thanks a lot!

FWIW:
Tested-by: Lino Sanfilippo <LinoSanfilippo@xxxxxx>

Best Regards,
Lino