drm: Hold the mode mutex whilst probing for sysfs status

commit 007c80a5 upstream.

As detect will use hw registers and may modify structures, it needs to be
serialised by use of the dev->mode_config.mutex. Make it so.

Otherwise, we may cause random crashes as the sysfs file is queried
whilst a concurrent hotplug poll is being run. For example:

[ 1189.189626] BUG: unable to handle kernel NULL pointer dereference at 00000100
[ 1189.189821] IP: [<e0c22019>] intel_tv_detect_type+0xa2/0x203 [i915]
[ 1189.190020] *pde = 00000000
[ 1189.190104] Oops: 0000 [#1] SMP
[ 1189.190209] last sysfs file: /sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-SVIDEO-1/status
[ 1189.190412] Modules linked in: mperf cpufreq_conservative cpufreq_userspace cpufreq_powersave cpufreq_stats decnet uinput fuse loop joydev snd_hd a_codec_realtek snd_hda_intel snd_hda_codec snd_hwdep snd_pcm_oss snd_mixer_oss snd_pcm i915 snd_seq_midi snd_rawmidi snd_seq_midi_event snd_seq drm_kms_helper snd_timer uvcvideo d rm snd_seq_device eeepc_laptop tpm_tis usbhid videodev i2c_algo_bit v4l1_compat snd sparse_keymap i2c_core hid serio_raw tpm psmouse evdev tpm_bios rfkill shpchp ac processor rng_c ore battery video power_supply soundcore pci_hotplug button output snd_page_alloc usb_storage uas ext3 jbd mbcache sd_mod crc_t10dif ata_generic ahci libahci ata_piix libata uhci_h cd ehci_hcd scsi_mod usbcore thermal atl2 thermal_sys nls_base [last unloaded: scsi_wait_scan]
[ 1189.192007]
[ 1189.192007] Pid: 1464, comm: upowerd Not tainted 2.6.37-2-686 #1 ASUSTeK Computer INC. 701/701
[ 1189.192007] EIP: 0060:[<e0c22019>] EFLAGS: 00010246 CPU: 0
[ 1189.192007] EIP is at intel_tv_detect_type+0xa2/0x203 [i915]
[ 1189.192007] EAX: 00000000 EBX: dca74000 ECX: e0f68004 EDX: 00068004
[ 1189.192007] ESI: dd110c00 EDI: 400c0c37 EBP: dca7429c ESP: de365e2c
[ 1189.192007]  DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068
[ 1189.192007] Process upowerd (pid: 1464, ti=de364000 task=dcc8acb0 task.ti=de364000)
[ 1189.192007] Stack: Mar 15 03:43:23 hostname kernel: [ 1189.192007]  e0c2cda4 70000000 400c0c30 00000000 dd111000 de365e54 de365f24 dd110c00
[ 1189.192007]  e0c22203 01000000 00000003 00000000 00000000 00000000 00000000 4353544e
[ 1189.192007]  30383420 00000069 00000000 00000000 00000000 00000000 00000000 00000000
[ 1189.192007] Call Trace: Mar 15 03:43:23 hostname kernel: [ 1189.192007]  [<e0c22203>] ?  intel_tv_detect+0x89/0x12d [i915]
[ 1189.192007]  [<e0a9dcef>] ?  status_show+0x0/0x2f [drm]
[ 1189.192007]  [<e0a9dd03>] ?  status_show+0x14/0x2f [drm]

[Digression: what is upowerd doing reading those power hungry files?]
Reported-by: Paul Menzel <paulepanter@users.sourceforge.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/gpu/drm/drm_sysfs.c

@@ -158,8 +158,15 @@ static ssize_t status_show(struct device *device,
 {
 	struct drm_connector *connector = to_drm_connector(device);
 	enum drm_connector_status status;
+	int ret;
+
+	ret = mutex_lock_interruptible(&connector->dev->mode_config.mutex);
+	if (ret)
+		return ret;
 
 	status = connector->funcs->detect(connector, true);
+	mutex_unlock(&connector->dev->mode_config.mutex);
+
 	return snprintf(buf, PAGE_SIZE, "%s\n",
 			drm_get_connector_status_name(status));
 }