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Subject4.10-rc2 oops in DRM connector code
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My Thinkpad x260 doesn't like to be unplugged from its dock.  I don't
think this is a new bug. It's happening on my distro's 4.4 kernel
as well.

The actual oops is in device_del(). It appears to have been passed a
null 'struct device *'.

There appears to have been a race _around_ here fixed in 1f7717552e.
I've looked for and tried to find the locking that prevents
drm_connector_unregister() from being called twice concurrently. I'm
unable to find anything.

drm_dp_destroy_connector_work() has some locking that looks useful:

mutex_lock(&mgr->destroy_connector_lock)

but it's released before the offending call:

mgr->cbs->destroy_connector(mgr, port->connector);

which actually calls intel_dp_destroy_mst_connector(). I have no idea
if it's correct (and haven't even run it with lockdep), but the attached
patch does seem to fix my oopses.

Any ideas?

> Jan 5 10:22:32 ray kernel: [ 537.087042] BUG: unable to handle kernel NULL pointer dereference at 000000000000009e
> Jan 5 10:22:32 ray kernel: [ 537.087954] IP: device_del+0x19/0x330
> Jan 5 10:22:32 ray kernel: [ 537.088860] PGD 0
> Jan 5 10:22:32 ray kernel: [ 537.088860]
> Jan 5 10:22:32 ray kernel: [ 537.090578] Oops: 0000 [#1] SMP
> Jan 5 10:22:32 ray kernel: [ 537.091406] Modules linked in: ctr ccm ipt_MASQUERADE nf_nat_masquerade_ipv4 iptable_nat nf_nat_ipv4 nf_nat nf_conntrack_ipv4 nf_defrag_ipv4 xt_conntrack nf_conntrack ipt_REJECT nf_reject_ipv4 xt_CHECKSUM iptable_mangle xt_tcpudp bridge stp llc iptable_filter ip_tables ebtable_nat ebtables x_tables cmac rfcomm bnep dm_crypt arc4 iwlmvm mac80211 snd_hda_codec_hdmi snd_hda_codec_realtek snd_hda_codec_generic iwlwifi intel_rapl snd_hda_intel iosf_mbi hid_logitech_hidpp snd_seq_midi cfg80211 x86_pkg_temp_thermal snd_hda_codec snd_seq_midi_event snd_hwdep btusb snd_rawmidi snd_hda_core btrtl coretemp snd_seq snd_pcm btbcm btintel joydev bluetooth ghash_clmulni_intel snd_timer shpchp thinkpad_acpi snd_seq_device nvram wmi snd soundcore mac_hid aesni_intel aes_x86_64 crypto_simd cryptd glue_helper kvm_intel
> Jan 5 10:22:32 ray kernel: [ 537.095222] kvm irqbypass hid_generic hid_logitech_dj usbhid hid
> Jan 5 10:22:32 ray kernel: [ 537.096272] CPU: 2 PID: 23 Comm: kworker/2:0 Tainted: G W 4.10.0-rc2 #47
> Jan 5 10:22:32 ray kernel: [ 537.097263] Hardware name: LENOVO 20F5S7V800/20F5S7V800, BIOS R02ET50W (1.23 ) 09/20/2016
> Jan 5 10:22:32 ray kernel: [ 537.098291] Workqueue: events drm_dp_destroy_connector_work
> Jan 5 10:22:32 ray kernel: [ 537.099328] task: ffff88040f2f1e00 task.stack: ffffc9000198c000
> Jan 5 10:22:32 ray kernel: [ 537.100335] RIP: 0010:device_del+0x19/0x330
> Jan 5 10:22:32 ray kernel: [ 537.101340] RSP: 0018:ffffc9000198fd58 EFLAGS: 00010282
> Jan 5 10:22:32 ray kernel: [ 537.102361] RAX: 0000000000000000 RBX: fffffffffffffffe RCX: ffff88040c5191b0
> Jan 5 10:22:32 ray kernel: [ 537.103418] RDX: ffffffff81cb6246 RSI: 0000000000000001 RDI: fffffffffffffffe
> Jan 5 10:22:32 ray kernel: [ 537.104473] RBP: ffffc9000198fd90 R08: 0000000000000000 R09: ffff880421517780
> Jan 5 10:22:32 ray kernel: [ 537.105574] R10: 0000007d0ce17c93 R11: 0000000000000001 R12: fffffffffffffffe
> Jan 5 10:22:32 ray kernel: [ 537.106636] R13: ffff88040ed36bd8 R14: ffff88040ed36788 R15: ffff88040c728810
> Jan 5 10:22:32 ray kernel: [ 537.107728] FS: 0000000000000000(0000) GS:ffff880421500000(0000) knlGS:0000000000000000
> Jan 5 10:22:32 ray kernel: [ 537.108812] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> Jan 5 10:22:32 ray kernel: [ 537.109937] CR2: 000000000000009e CR3: 0000000384894000 CR4: 00000000003406e0
> Jan 5 10:22:32 ray kernel: [ 537.111038] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> Jan 5 10:22:32 ray kernel: [ 537.112142] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Jan 5 10:22:32 ray kernel: [ 537.113223] Call Trace:
> Jan 5 10:22:32 ray kernel: [ 537.114293] device_unregister+0x12/0x30
> Jan 5 10:22:32 ray kernel: [ 537.115354] drm_sysfs_connector_remove+0x3b/0x50
> Jan 5 10:22:32 ray kernel: [ 537.116391] drm_connector_unregister.part.8+0x27/0x40
> Jan 5 10:22:32 ray kernel: [ 537.117433] drm_connector_unregister+0x14/0x20
> Jan 5 10:22:32 ray kernel: [ 537.118478] intel_dp_destroy_mst_connector+0x1a/0x80
> Jan 5 10:22:32 ray kernel: [ 537.119513] drm_dp_destroy_connector_work+0xa9/0x150
> Jan 5 10:22:32 ray kernel: [ 537.120539] process_one_work+0x14b/0x430
> Jan 5 10:22:32 ray kernel: [ 537.121568] worker_thread+0x12b/0x4a0
> Jan 5 10:22:32 ray kernel: [ 537.122581] kthread+0x10c/0x140
> Jan 5 10:22:32 ray kernel: [ 537.123583] ? process_one_work+0x430/0x430
> Jan 5 10:22:32 ray kernel: [ 537.124584] ? kthread_create_on_node+0x40/0x40
> Jan 5 10:22:32 ray kernel: [ 537.125574] ret_from_fork+0x27/0x40
> Jan 5 10:22:32 ray kernel: [ 537.126562] Code: 00 00 00 00 00 00 00 5b 41 5c 41 5d 5d c3 0f 1f 40 00 0f 1f 44 00 00 55 48 89 e5 41 57 41 56 41 55 41 54 49 89 fc 53 48 83 ec 10 <48> 8b 87 a0 00 00 00 4c 8b 2f 48 85 c0 74 1b 48 8b b8 90 00 00
> Jan 5 10:22:32 ray kernel: [ 537.127644] RIP: device_del+0x19/0x330 RSP: ffffc9000198fd58
> Jan 5 10:22:32 ray kernel: [ 537.128690] CR2: 000000000000009e
> Jan 5 10:22:32 ray kernel: [ 537.129759] ---[ end trace 7e17c77627e8f513 ]---
diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index aa64448..85beebc 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -2914,13 +2914,14 @@ static void drm_dp_destroy_connector_work(struct work_struct *work)
break;
}
list_del(&port->next);
- mutex_unlock(&mgr->destroy_connector_lock);

kref_init(&port->kref);
INIT_LIST_HEAD(&port->next);

mgr->cbs->destroy_connector(mgr, port->connector);

+ mutex_unlock(&mgr->destroy_connector_lock);
+
drm_dp_port_teardown_pdt(port, port->pdt);
port->pdt = DP_PEER_DEVICE_NONE;
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