Hi, On 01-06-17 14:14, Chris Wilson wrote:
On Thu, Jun 01, 2017 at 01:54:30PM +0200, Hans de Goede wrote: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.That really does sound like a step in the wrong direction though. But that seems to because of the call to driver->unload() from the middle of unregister that is catching me by surprise. > Regression fix trumps niceties, so
Right, I'm not entirely happy with this fix too, but I decided to go for the simplest fix possible to get the regression fixed. As you've seen in my reply in the "[PATCH v11] drm: Unplug drm device when unregistering it (v8)" thread I have been thinking about nicer ways to fix this too. Lets continue the discussion about doing a better fix for a future kernel release there.
Reviewed-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Thank you.
Looks like about 50% of the remaining driver->unload callbacks can be replaced by a driver->release callback. The rest need care to finish demidlayering driver->load/unload. And I still think unplug needs some polish.
Regards, Hans