Re: [PATCH] component: try_module_get() to prevent unloading while in use

[Richard Fitzgerald <rf@opensource.cirrus.com>]

On 26/07/2022 18:28, Greg KH wrote:
> On Tue, Jul 26, 2022 at 11:32:28AM +0100, Richard Fitzgerald wrote:
>> On 25/07/2022 19:09, Greg KH wrote:
>>> On Mon, Jul 25, 2022 at 05:08:59PM +0100, Richard Fitzgerald wrote:
>>>> Call try_module_get() on a component before attempting to call its
>>>> bind() function, to ensure that a loadable module cannot be
>>>> unloaded while we are executing its bind().
>>>
>>> How can bind be called while the module is unloaded?
>>>
>>
>> I didn't say it could. What I said is "unloaded while we are executing
>> its bind()". Maybe that's already guaranteed to be safe somehow. It's
>> actually the problem below that I was trying to fix but placing the
>> try_module_get() before the bind() rather than after bind() seemed a
>> trivial extra safety.
> 
> It should be safe, bind() can't race with module remove as the driver
> core locks will handle this.
> 
>>>> If the bind is successful the module_put() is called only after it
>>>> has been unbound. This ensures that the module cannot be unloaded
>>>> while it is in use as an aggregate device.
>>>
>>> That's almost never the correct thing to do, what problem is this
>>> solving?
>>>
>>
>> What I see is that when a loadable module has been made part of an
>> aggregate it is still possible to rmmod'd it.
>>
>> An alternative workaround would be for the parent to softdep to every
>> driver that _might_ provide the aggregated components. Softdeps aren't
>> unusual (we use it in some drivers that are directly related but don't
>> directly link into each other). But to me this feels like a hack when
>> used with the component framework - isn't the idea that the parent
>> doesn't know (or doesn't need to know) which drivers will be aggregated?
>> Wouldn't it be better that when a component driver is bound into an
>> aggregate its module is automatically marked in-use?
>>
>> If there's a better way to mark the module in-use while is it bound
>> into an aggregate, let me know and I'll look at implementing it.
> 
> No module references should be incremented if a device is bound to a
> driver, that's the old (1990's) way of thinking.  If a module wants to
> be unloaded, let it, and clean up everything that it was
> controlling/talking to before the module remove is finished.
> 
> That's the way all busses should be working, you don't increment a
> module count when a driver binds to a device, otherwise how would you
> unload a module that was being used at all?
> 
> So just remove the components controlled by the module properly when it
> is removed and all should be good.
> 
> Do you have example code in the kernel tree today that does not properly
> do this?  Why not just fix that instead?
> 

The actual code I'm working on isn't upstream yet, but it's a derivative
of the way these two interoperate:
  sound/pci/hda/patch_realtek
  sound/pci/hda/cs35l41_hda.c

In these systems the host audio interface is HDA but the amps are not
HDA devices. Audio goes through the Realtek HDA codec to the amp but
the amp is on a different bus (i2c or spi). The modules in the HDA stack
all get marked as in-use, but the amp driver doesn't. So if it's
unloaded the audio system is left in a limbo state where ALSA and the
HDA stack are still up but the amp driver code has gone.

However I realised that my try_module_get() isn't a fix anyway.
It's claiming use of the module implementing the component but not
of the bus that owns that module. I assume that's what you were
referring to by having to deal with an unload instead of trying to
prevent the unload.

(And yes, I'm aware that in that patch_realtek.c it's missing
locking around the shared struct to prevent it being accessed during a
bind and unbind.)

> thanks,
> 
> greg k-h