Re: [PATCH v3 2/9] HID: haptic: Get rid of input_ff_destroy() in error paths

[Dmitry Torokhov]

Hi Rong,

On Fri, Nov 13, 2026 at 01:53:55AM +0800, Rong Zhang wrote:
> It's painful to have error paths after calling input_ff_create(),
> because input_ff_destroy() frees too much things but does not restore
> some other things.
>
> On get_device failure, the module's bail-out path calls module_put()
> right before hid_haptic_destroy() calling it again, causing refcount
> underflow.
>
> Besides, every error path calling input_ff_destroy() frees ff->private,
> while the others don't. This divergence has caused quite a lot of
> trouble.
>
> Get the module and the device beforehand, so that there is no error path
> after input_ff_create(), eliminating the call to input_ff_destroy(). In
> this manner, we don't need to restore dev->flush and dev->event either.

The only reason hid_haptic_init() had try_module_get(THIS_MODULE) and
get_device(&hdev->dev) (and their counterparts in hid_haptic_destroy())
was to work around input_ff_destroy() previously being called only from
input_dev_release(), which could be deferred until after hdev was
unbound or the driver module was unloaded if userspace kept the evdev
node open.

I am queuing an input core change ("Input: destroy force-feedback device
on input unregistration") [1] that calls input_ff_destroy() directly in
__input_unregister_device() once all handlers are disconnected and the
device is removed from sysfs.

With that change in place, hid_haptic_destroy() is guaranteed to run
synchronously during input_unregister_device() while hdev is still bound
and alive (and dev->dev.parent already holds a reference to &hdev->dev
anyway). That means get_device(&hdev->dev) / put_device(&hdev->dev) and
try_module_get(THIS_MODULE) / module_put(THIS_MODULE) are not needed at
all here (until patch 8/9 where hid-haptic becomes a separate plugin
module and hid_plugin_get() is taken before calling self_plugin->init()),
and simply dropping them eliminates the error path after
input_ff_create(). It also avoids the devres lifetime issue with
put_device(&hdev->dev) mentioned in patch 1/9.

[1] https://lore.kernel.org/linux-input/2f722967b6db0692d4dd4164aff39b62@xxxxxxxxx/

Thanks.

-- 
Dmitry