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

[Richard Fitzgerald]

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