Re: [PATCH v11] drm: Unplug drm device when unregistering it (v8)

[Hans de Goede <hdegoede@redhat.com>]

Hi,

On 29-05-17 22:25, Chris Wilson wrote:
> On Fri, Apr 14, 2017 at 11:15:04AM -0400, Sean Paul wrote:
>> On Thu, Apr 13, 2017 at 03:32:44PM +0800, Jeffy Chen wrote:
>>> After unbinding drm, the user space may still owns the drm dev fd, and
>>> may still be able to call drm ioctl.
>>>
>>> We're using an unplugged state to prevent something like that, so let's
>>> reuse it here.
>>>
>>> Also drop drm_unplug_dev, because it would be unused after other changes.
>>>
>>> Verified on rk3399 chromebook kevin(with cros 4.4 kernel), no more crashes
>>> when unbinding drm with ui service still running.
>>>
>>> v2: Fix some commit messages.
>>> v3: Reuse unplug status.
>>> v4: Add drm_device_set_plug_state helper.
>>> v5: Fix hang when unregistering drm dev with open_count 0.
>>> v6: Move drm_device_set_plug_state into drm_drv.
>>> v7: Add missing drm_dev_unref in udl_drv.
>>> v8: Fix compiler errors after enable udl.
>>>
>>> Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
>>>
>>> ---
>>
>> Hi Jeffy,
>> Given the trouble we've had with this patch already, coupled with the fact that
>> unbinding while userspace is active is a contrived/pathological case, I don't
>> think it's worth picking this patch upstream.
>>
>> If it's really causing issues downstream, you can add my Reviewed-by for a CHROMIUM
>> patch, but I'd rather not carry patches in the CrOS repo if we don't need to.
> 
> Would a
> 
> Fixes: a39be606f99d ("drm: Do a full device unregister when unplugging")
> Reported-by: Marco Diego Aurélio Mesquita <marcodiegomesquita@gmail.com>
> Cc: Hans de Goede <hdegoede@redhat.com>
> 
> convince us to look into this patch again?

The problem is this patch is wrong, see below.

>>>   drivers/gpu/drm/drm_drv.c     | 26 ++++++++++----------------
>>>   drivers/gpu/drm/udl/udl_drv.c |  3 ++-
>>>   include/drm/drmP.h            |  6 ------
>>>   include/drm/drm_drv.h         |  1 -
>>>   4 files changed, 12 insertions(+), 24 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
>>> index b5c6bb4..e1da4d1 100644
>>> --- a/drivers/gpu/drm/drm_drv.c
>>> +++ b/drivers/gpu/drm/drm_drv.c
>>> @@ -355,22 +355,6 @@ void drm_put_dev(struct drm_device *dev)
>>>   }
>>>   EXPORT_SYMBOL(drm_put_dev);
>>>   
>>> -void drm_unplug_dev(struct drm_device *dev)
>>> -{
>>> -	/* for a USB device */
>>> -	drm_dev_unregister(dev);
>>> -
>>> -	mutex_lock(&drm_global_mutex);
>>> -
>>> -	drm_device_set_unplugged(dev);
>>> -
>>> -	if (dev->open_count == 0) {
>>> -		drm_put_dev(dev);
>>> -	}
>>> -	mutex_unlock(&drm_global_mutex);
>>> -}
>>> -EXPORT_SYMBOL(drm_unplug_dev);
>>> -
>>>   /*
>>>    * DRM internal mount
>>>    * We want to be able to allocate our own "struct address_space" to control
>>> @@ -733,6 +717,13 @@ static void remove_compat_control_link(struct drm_device *dev)
>>>   	kfree(name);
>>>   }
>>>   
>>> +static inline void drm_device_set_plug_state(struct drm_device *dev,
>>> +					     bool plugged)
>>> +{
>>> +	smp_wmb();
>>> +	atomic_set(&dev->unplugged, !plugged);
>>> +}
>>> +
>>>   /**
>>>    * drm_dev_register - Register DRM device
>>>    * @dev: Device to register
>>> @@ -787,6 +778,8 @@ int drm_dev_register(struct drm_device *dev, unsigned long flags)
>>>   	if (drm_core_check_feature(dev, DRIVER_MODESET))
>>>   		drm_modeset_register_all(dev);
>>>   
>>> +	drm_device_set_plug_state(dev, true);
>>> +
>>>   	ret = 0;
>>>   
>>>   	DRM_INFO("Initialized %s %d.%d.%d %s for %s on minor %d\n",
>>> @@ -826,6 +819,7 @@ void drm_dev_unregister(struct drm_device *dev)
>>>   	drm_lastclose(dev);
>>>   
>>>   	dev->registered = false;
>>> +	drm_device_set_plug_state(dev, false);
>>>   
>>>   	if (drm_core_check_feature(dev, DRIVER_MODESET))
>>>   		drm_modeset_unregister_all(dev);
>>> diff --git a/drivers/gpu/drm/udl/udl_drv.c b/drivers/gpu/drm/udl/udl_drv.c
>>> index cd8b017..fc73e24 100644
>>> --- a/drivers/gpu/drm/udl/udl_drv.c
>>> +++ b/drivers/gpu/drm/udl/udl_drv.c
>>> @@ -108,7 +108,8 @@ static void udl_usb_disconnect(struct usb_interface *interface)
>>>   	drm_kms_helper_poll_disable(dev);
>>>   	udl_fbdev_unplug(dev);
>>>   	udl_drop_usb(dev);
>>> -	drm_unplug_dev(dev);
>>> +	drm_dev_unregister(dev);
>>> +	drm_dev_unref(dev);

The unref here will cause the device struct to get free-ed even if
userspace still holds references to it through the drm_dev. To fix
this we would need to call drm_dev_ref on a open from userspace and
drm_dev_unref from drm_release.

Regards,

Hans




>>>   }
>>>   
>>>   /*
>>> diff --git a/include/drm/drmP.h b/include/drm/drmP.h
>>> index 3bfafcd..980a204 100644
>>> --- a/include/drm/drmP.h
>>> +++ b/include/drm/drmP.h
>>> @@ -488,12 +488,6 @@ static __inline__ int drm_core_check_feature(struct drm_device *dev,
>>>   	return ((dev->driver->driver_features & feature) ? 1 : 0);
>>>   }
>>>   
>>> -static inline void drm_device_set_unplugged(struct drm_device *dev)
>>> -{
>>> -	smp_wmb();
>>> -	atomic_set(&dev->unplugged, 1);
>>> -}
>>> -
>>>   static inline int drm_device_is_unplugged(struct drm_device *dev)
>>>   {
>>>   	int ret = atomic_read(&dev->unplugged);
>>> diff --git a/include/drm/drm_drv.h b/include/drm/drm_drv.h
>>> index 0fefc3f..eb63078 100644
>>> --- a/include/drm/drm_drv.h
>>> +++ b/include/drm/drm_drv.h
>>> @@ -544,7 +544,6 @@ void drm_dev_unregister(struct drm_device *dev);
>>>   void drm_dev_ref(struct drm_device *dev);
>>>   void drm_dev_unref(struct drm_device *dev);
>>>   void drm_put_dev(struct drm_device *dev);
>>> -void drm_unplug_dev(struct drm_device *dev);
>>>   
>>>   int drm_dev_set_unique(struct drm_device *dev, const char *name);
>