Patch "drm: Fix oops + Xserver hang when unplugging USB drm devices" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

drm: Fix oops + Xserver hang when unplugging USB drm devices

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
drm-fix-oops-xserver-hang-when-unplugging-usb-drm-devices.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Hans de Goede <[email protected]>
Date: Thu, 1 Jun 2017 13:54:30 +0200
Subject: drm: Fix oops + Xserver hang when unplugging USB drm devices
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

From: Hans de Goede <[email protected]>

commit 75fb636324a839c2c31be9f81644034c6142e469 upstream.

commit a39be606f99d ("drm: Do a full device unregister when unplugging") causes backtraces like this one when unplugging an usb drm device while it is in use:

usb 2-3: USB disconnect, device number 25

------------[ cut here ]------------
WARNING: CPU: 0 PID: 242 at drivers/gpu/drm/drm_mode_config.c:424
   drm_mode_config_cleanup+0x220/0x280 [drm]
...
RIP: 0010:drm_mode_config_cleanup+0x220/0x280 [drm]
...
Call Trace:
 gm12u320_modeset_cleanup+0xe/0x10 [gm12u320]
 gm12u320_driver_unload+0x35/0x70 [gm12u320]
 drm_dev_unregister+0x3c/0xe0 [drm]
 drm_unplug_dev+0x12/0x60 [drm]
 gm12u320_usb_disconnect+0x36/0x40 [gm12u320]
 usb_unbind_interface+0x72/0x280
 device_release_driver_internal+0x158/0x210
 device_release_driver+0x12/0x20
 bus_remove_device+0x104/0x180
 device_del+0x1d2/0x350
 usb_disable_device+0x9f/0x270
 usb_disconnect+0xc6/0x260
...
[drm:drm_mode_config_cleanup [drm]] *ERROR* connector Unknown-1 leaked!
------------[ cut here ]------------
WARNING: CPU: 0 PID: 242 at drivers/gpu/drm/drm_mode_config.c:458
   drm_mode_config_cleanup+0x268/0x280 [drm]
...
<same Call Trace>
---[ end trace 80df975dae439ed6 ]---
general protection fault: 0000 [#1] SMP
...
Call Trace:
 ? __switch_to+0x225/0x450
 drm_mode_rmfb_work_fn+0x55/0x70 [drm]
 process_one_work+0x193/0x3c0
 worker_thread+0x4a/0x3a0
...
RIP: drm_framebuffer_remove+0x62/0x3f0 [drm] RSP: ffffb776c39dfd98
---[ end trace 80df975dae439ed7 ]---

After which the system is unusable this is caused by drm_dev_unregister getting called immediately on unplug, which calls the drivers unload function which calls drm_mode_config_cleanup which removes the framebuffer object while userspace is still holding a reference to it.

Reverting commit a39be606f99d ("drm: Do a full device unregister when unplugging") leads to the following oops on unplug instead, when userspace closes the last fd referencing the drm_dev:

sysfs group 'power' not found for kobject 'card1-Unknown-1'

------------[ cut here ]------------
WARNING: CPU: 0 PID: 2459 at fs/sysfs/group.c:237
   sysfs_remove_group+0x80/0x90
...
RIP: 0010:sysfs_remove_group+0x80/0x90
...
Call Trace:
 dpm_sysfs_remove+0x57/0x60
 device_del+0xfd/0x350
 device_unregister+0x1a/0x60
 drm_sysfs_connector_remove+0x39/0x50 [drm]
 drm_connector_unregister+0x5a/0x70 [drm]
 drm_connector_unregister_all+0x45/0xa0 [drm]
 drm_modeset_unregister_all+0x12/0x30 [drm]
 drm_dev_unregister+0xca/0xe0 [drm]
 drm_put_dev+0x32/0x60 [drm]
 drm_release+0x2f3/0x380 [drm]
 __fput+0xdf/0x1e0
...
---[ end trace ecfb91ac85688bbe ]---
BUG: unable to handle kernel NULL pointer dereference at 00000000000000a8
IP: down_write+0x1f/0x40
...
Call Trace:
 debugfs_remove_recursive+0x55/0x1b0
 drm_debugfs_connector_remove+0x21/0x40 [drm]
 drm_connector_unregister+0x62/0x70 [drm]
 drm_connector_unregister_all+0x45/0xa0 [drm]
 drm_modeset_unregister_all+0x12/0x30 [drm]
 drm_dev_unregister+0xca/0xe0 [drm]
 drm_put_dev+0x32/0x60 [drm]
 drm_release+0x2f3/0x380 [drm]
 __fput+0xdf/0x1e0
...
---[ end trace ecfb91ac85688bbf ]---

This is caused by the revert moving back to drm_unplug_dev calling drm_minor_unregister which does:

        device_del(minor->kdev);
        dev_set_drvdata(minor->kdev, NULL); /* safety belt */
        drm_debugfs_cleanup(minor);

Causing the sysfs entries to already be removed even though we still have references to them in e.g. drm_connector.

Note we must call drm_minor_unregister to notify userspace of the unplug of the device, so calling drm_dev_unregister is not completely wrong the problem is that drm_dev_unregister does too much.

This commit fixes drm_unplug_dev by not only reverting commit a39be606f99d ("drm: Do a full device unregister when unplugging") but by also adding a call to drm_modeset_unregister_all before the drm_minor_unregister calls to make sure all sysfs entries are removed before calling device_del(minor->kdev) thereby also fixing the second set of oopses caused by just reverting the commit.

---
 drivers/gpu/drm/drm_drv.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

@@ -379,7 +379,12 @@ EXPORT_SYMBOL(drm_put_dev);
 void drm_unplug_dev(struct drm_device *dev)
 {
 	/* for a USB device */
-	drm_dev_unregister(dev);
+	if (drm_core_check_feature(dev, DRIVER_MODESET))
+		drm_modeset_unregister_all(dev);
+
+	drm_minor_unregister(dev, DRM_MINOR_PRIMARY);
+	drm_minor_unregister(dev, DRM_MINOR_RENDER);
+	drm_minor_unregister(dev, DRM_MINOR_CONTROL);
 
 	mutex_lock(&drm_global_mutex);
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/drm-fix-oops-xserver-hang-when-unplugging-usb-drm-devices.patch