[PATCH 1/9] drm/connector: Make the drm_sysfs connector->kdev device hold a reference to the connector

[Hans de Goede]

Hi,

On 4/29/21 2:04 PM, Daniel Vetter wrote:
> On Thu, Apr 29, 2021 at 01:54:46PM +0200, Greg Kroah-Hartman wrote:
>> On Thu, Apr 29, 2021 at 01:40:28PM +0200, Daniel Vetter wrote:
>>> On Wed, Apr 28, 2021 at 11:52:49PM +0200, Hans de Goede wrote:
>>>> Userspace could hold open a reference to the connector->kdev device,
>>>> through e.g. holding a sysfs-atrtribute open after
>>>> drm_sysfs_connector_remove() has been called. In this case the connector
>>>> could be free-ed while the connector->kdev device's drvdata is still
>>>> pointing to it.
>>>>
>>>> Give drm_connector devices there own device type, which allows
>>>> us to specify our own release function and make drm_sysfs_connector_add()
>>>> take a reference on the connector object, and have the new release
>>>> function put the reference when the device is released.
>>>>
>>>> Giving drm_connector devices there own device type, will also allow
>>>> checking if a device is a drm_connector device with a
>>>> "if (device->type == &drm_sysfs_device_connector)" check.
>>>>
>>>> Note that the setting of the name member of the device_type struct will
>>>> cause udev events for drm_connector-s to now contain DEVTYPE=drm_connector
>>>> as extra info. So this extends the uevent part of the userspace API.
>>>>
>>>> Signed-off-by: Hans de Goede <hdegoede at redhat.com>
>>>
>>> Are you sure? I thought sysfs is supposed to flush out any pending
>>> operations (they complete fast) and handle open fd internally?
>>
>> Yes, it "should" :)
> 
> Thanks for confirming my vague memories :-)
> 
> Hans, pls drop this one.

Please see my earlier reply to your review of this patch, it is
still needed but for a different reason:

"""
We still need this change though to make sure that the 
"drm/connector: Add drm_connector_find_by_fwnode() function"
does not end up following a dangling drvdat pointer from one
if the drm_connector kdev-s.

The class_dev_iter_init() in drm_connector_find_by_fwnode() gets
a reference on all devices and between getting that reference
and it calling drm_connector_get() - drm_connector_unregister()
may run and drop the possibly last reference to the
drm_connector object, freeing it and leaving the kdev's
drvdata as a dangling pointer.
"""

This is actually why I added it initially, and while adding it
I came up with this wrong theory of why it was necessary independently
of the drm_connector_find_by_fwnode() addition, sorry about that.

Regards,

Hans