RE: [EXT] Re: [PATCH] usb: roles: try to get/put all relevant modules

Xu Yang
2024-01-17T05:57:05Z

Hi Alan,

>
> On Tue, Jan 16, 2024 at 05:44:47AM +0000, Xu Yang wrote:
> > Hi Alan,
> >
> > >
> > > Those of us unfamiliar with this code need you to explain a lot more
> > > about what's going on.
> > >
> > > On Mon, Jan 15, 2024 at 03:02:06AM +0000, Xu Yang wrote:
> > > > Taking below diagram as example:
> > > >
> > > > ci_hdrc.0 register usb get tcpm_port
> > > > (driver: ci_hdrc) ---------> role <---- (driver: tcpm)
> > > > ^ ^ switch | ^
> > > > | | | |
> > > > +1| | +1 | |+1
> > > > | +-------------------------------------+ |
> > > > | |
> > > > 4c200000.usb 1-0050
> > > > (driver: ci_hdrc_imx) (driver: tcpci)
> > > >
> > > > 1. Driver ci_hdrc_imx and tcpci are built as module at least.
> > > > 2. When module ci_hdrc_imx is loaded, it will register ci_hdrc.0 device
> > > > and try to get ci_hdrc module's reference.
> > >
> > > This is very confusing. Normally, a device is registered by the parent
> > > module and its driver belongs in the child module. When the child
> > > module is loaded it automatically gets a reference to the parent module,
> > > because it calls functions that are defined in the parent. I don't know
> > > of any cases where a parent module takes a reference to one of its
> > > children -- this would make it impossible to unload the child module!
> > >
> > > In your diagram I can't tell whether ci_hdrc is the parent module and
> > > ci_hdrc_imx is the child, or vice versa. I'll guess that ci_hdrc_imx is
> > > the child, since it the one which gets a reference to the other. But
> > > now we have the ci_hdrc.0 device being registered by the child module
> > > and its driver belonging to the parent module, which is backward!
> > >
> > > Very difficult to understand. Please explain more fully.
> >
> > I checked again and let me correct the words.
> >
> > 2. When module ci_hdrc_imx is loaded, it will register ci_hdrc.0 device.
> > At the same time, the reference of module ci_hdrc is added by 1
> > automatically due to ci_hdrc_imx calls some functions in module ci_hdrc.
> > ci_hdrc will register usb-role-switch device.
> >
> > Therefore, module ci_hdrc_imx depends on module ci_hdrc. Device ci_hdrc.0
> > is a child of 4c200000.usb.
>
> And ci_hdrc_imx is a child module of ci_hdrc. Got it.
>
> > > > ci_hdrc will register
> > > > usb-role-switch device.
> > > > 3. When module tcpci is loaded, it will register tcpm port device and try
> > > > to get tcpm module's reference. The tcpm module will get usb-role-switch
> > > > which is registered by ci_hdrc.
> > >
> > > What do you mean by "will get"? Do you mean that tcpm will become the
> > > driver for the usb_role_switch device? Or do you mean that it simply
> > > calls get_device(&usb_role_switch)?
> > >
> > > If the latter is the case, how does the tcpm driver learn the address of
> > > usb_role_switch in the first place?
> >
> > Via
> > port->role_sw = usb_role_switch_get(port->dev)
> > or
> > port->role_sw = fwnode_usb_role_switch_get(tcpc->fwnode).
> >
> > The usb controller will register usb-role-swtich device to the global list
> > of usb_role class. The fwnode of usb-role-swtich device is also set to usb
> > controller's fwnode. Initially, a fwnode graph between usb controller of
> > node and tcpm connector node had already been established. These two
> > functions will find usb-role-swtich device based on this fwnode graph
> > and fwnode matching.
>
> If usb_role_switch_get() gives away references to the usb_role_switch
> device, it should have a way to take those references back. But I guess
> it doesn't.
>
> > After usb-role-switce device is found, these two
> > functions will call: try_module_get(sw->dev.parent->driver->owner).
>
> You mean usb_role_switch_get() and fwnode_usb_role_switch_get() do this?

Yes.

>
> > Here sw->dev.parent is device ci_hdrc.0. sw->dev.parent->driver is ci_hdrc.
> >
> > >
> > > > In current design, tcpm will also try to
> > > > get ci_hdrc module's reference after get usb-role-switch.
> > >
> > > This might be a bug. There should not be any need for the tcpm driver
> > > to take a reference to the ci_hdrc module. But there should be a way
> > > for the ci_hdrc driver to notify tcpm when the usb_role_switch device is
> > > about to be unregistered. If tcpm is usb_role_switch's driver then this
> > > notification happens automatically, by means of the .remove() callback.
> >
> > I'm not the designer of usb_role class driver. Not sure if this is needed to get
> > module reference of its parent device's driver. Maybe need @heikki's input.
> >
> > @heikki.krogerus, can you give some explanations?
>
> Yes, please, some additional explanation would help.
>
> > > > 4. Due to no modules depend on ci_hdrc_imx, ci_hdrc_imx can be manually
> > > > unloaded. Then device ci_hdrc.0 will be removed by ci_hdrc_imx and
> > > > device usb-role-switch is also unregistered.
> > >
> > > At this point, tcpm should learn that it has to drop all its references
> > > to usb_role_swich. Since the module which registered usb_role_switch
> > > isn't tcpm's ancestor, tcpm must not keep _any_ references to the device
> > > after it is unregistered.
> >
> > Yes, I also think so.
> >
> > >
> > > Well, strictly speaking that's not true. By misusing the driver model,
> > > tcpm could keep a reference to the ci_hdrc module until it was finished
> > > using usb_role_switch. Is that what you are trying to do?
> >
> > No, I'm trying to get module reference of ci_hdrc_imx too. Then,
> > ci_hdrc_imx can't be unloaded before tcpci module unloaded.
>
> You shouldn't do this. Users should be able to unload ci_hdrc_imx
> whenever they want, even if tcpci is still loaded.

Okay. Understand.

>
> > > > 5. Then, if I try to unload module tcpci, "NULL pointer dereference"
> > > > will be shown due to below code:
> > > >
> > > > module_put(sw->dev.parent->driver->owner);
>
> I forgot to ask: What function makes this call? Is it part of the
> usb_role class driver?

usb_role_switch_put() do this.
Yes, it's a function of usb_role class driver.

>
> > > > parent->driver is NULL at this time.
> > >
> > > What is dev at this point? And what is dev.parent? And what did
> > > dev.parent->driver used to be before it was set to NULL?
> >
> > Here sw->dev is usb-role-switch device. sw->dev.parent is ci_hdrc.0 device.
> > sw->dev.parent->driver was ci_hdrc.
>
> Which is now gone, right. I understand.
>
> Let's see what Heikki has to say.
>
> However, assuming he wants to continue misusing the driver model in this
> way, what you should do is add a new field to sw, where you will store
> sw->dev.parent->driver.owner at the time of the try_module_get() call
> (but only if the call succeeds!). Then when the module_put() call runs,
> have it use the value stored in this new field instead of dereferencing
> sw->dev.parent->driver.owner.

It sounds like a better solution.
Thanks for the suggestion!

Best Regards,
Xu Yang