Re: [PATCH net-next 06/16] net: pcs: xpcs: Avoid creating dummy XPCS MDIO device

Serge Semin
2023-12-13T15:47:40Z

On Tue, Dec 12, 2023 at 07:06:46PM +0000, Russell King (Oracle) wrote:
> On Tue, Dec 12, 2023 at 06:26:16PM +0300, Serge Semin wrote:
> > I would have used in the first place if it was externally visible, but
> > it's defined as static. Do you suggest to make it global or ...
>
> That would be one option - I didn't make it visible when I introduced it
> beacuse there were no users for it.
>
> > > At some point, we should implement
> > > mdiobus_get_mdiodev() which also deals with the refcount.
> >
> > ... create mdiobus_get_mdiodev() instead?
> >
> > * Note in the commit message I mentioned that having a getter would be
> > * better than directly touching the mii_bus instance guts.
>
> What I'm thinking is:
>
> /**
> * mdiobus_get_mdiodev() - get a mdiodev for the specified bus
> * @bus: mii_bus to get mdio device from
> * @addr: mdio address of mdio device
> *
> * Return the struct mdio_device attached to the MII bus @bus at MDIO
> * address @addr. On success, the refcount on the device will be
> * increased, which must be dropped using mdio_device_put(), and the
> * mdio device returned. Otherwise, returns NULL.
> */
> struct mdio_device *mdiobus_get_mdiodev(struct mii_bus *bus, int addr)
> {
> struct mdio_device *mdiodev;
>
> mdiodev = mdiobus_find_device(bus, addr);
> if (mdiodev)
> get_device(&mdiodev->dev);
> return mdiodev;
> }
> EXPORT_SYMBOL(mdiobus_get_mdiodev);
>
> should do it, and will hold a reference on the mdiodev structure (which
> won't be freed) and also on the mii_bus (since this device is a child
> of the bus device, the parent can't be released until the child has
> been, so struct mii_bus should at least stay around.)

Right. That's exactly what had in mind. Thanks for suggesting a
ready-to-apply solution. I'll add it to the series as a separate patch
if we decide to keep the proposed in this patch change. See my
question in the next message:
https://lore.kernel.org/netdev/wnptneaxxe2tq2rf7ac6a72xtyluyggughvmtxbbg5qto64mpa@7gchl5e4qllu/

>
> What would help the "the bus driver has been unbound" situation is if
> we took the mdio_lock on the bus, and then set the {read,write}{,_c45}
> functions to dummy stubs when the bus is being unregistered which then
> return e.g. -ENXIO. That will probably make unbinding/unloading all
> MDIO bus drivers safe from kernel oops, although phylib will spit out
> a non-useful backtrace if it tries an access. I don't think there's
> much which can be done about that - I did propose a patch to change
> that behaviour but apparently folk like having it!
>
> It isn't perfect - it's racy, but then accessing mdio_map[] is
> inherently racy due to no locking with mdiobus_.*register_device().
> At least if we have everyone using a proper getter function rather
> than directly fiddling with bus->mdio_map[]. We only have one driver
> that accesses it directly at the moment (mscc_ptp):
>
> dev = phydev->mdio.bus->mdio_map[vsc8531->ts_base_addr];
> phydev = container_of(dev, struct phy_device, mdio);
>
> return phydev->priv;
>
> and that should really be using mdiobus_get_phy().

Regarding the driver bind/unbind. I guess the maintainers just forget
about that problem. Do you think it's worth reminding them about it?

-Serge(y)

>
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