drm/i915/gt: Check for a registered driver with IPS

If the ips module calls into the driver during an unbind/bind cycle, we
may see the driver while it has unregistered itself from ips and try and
dereference a NULL ips_mchdev pointer.

<1> [211.928844] BUG: kernel NULL pointer dereference, address: 0000000000000014
<1> [211.928861] #PF: supervisor read access in kernel mode
<1> [211.928871] #PF: error_code(0x0000) - not-present page
<6> [211.928881] PGD 0 P4D 0
<4> [211.928890] Oops: 0000 [#1] PREEMPT SMP PTI
<4> [211.928900] CPU: 3 PID: 327 Comm: ips-monitor Not tainted 5.9.0-rc5-CI-CI_DRM_9008+ #1
<4> [211.928914] Hardware name: Hewlett-Packard HP EliteBook 8440p/172A, BIOS 68CCU Ver. F.24 09/13/2013
<4> [211.929056] RIP: 0010:mchdev_get+0x5a/0x180 [i915]
<4> [211.929067] Code: c0 5a 74 0d 80 3d f1 53 29 00 00 0f 84 ab 00 00 00 48 8b 1d c8 a8 29 00 e8 d3 18 89 e1 85 c0 74 09 80 3d d1 53 29 00 00 74 65 <8b> 4b 14 48 8d 7b 14 85 c9 0f 84 09 01 00 00 8d 51 01 89 c8 f0 0f
<4> [211.929095] RSP: 0018:ffffc900002efe60 EFLAGS: 00010202
<4> [211.929105] RAX: 0000000000000001 RBX: 0000000000000000 RCX: ffff8881297acf40
<4> [211.929118] RDX: 0000000000000000 RSI: ffffffff8264e2c0 RDI: ffff8881297ad820
<4> [211.929130] RBP: ffffc900002efe68 R08: ffff8881297ad820 R09: 00000000fffffffe
<4> [211.929143] R10: ffff8881297acf40 R11: 00000000fff74c96 R12: ffff8881294dfa18
<4> [211.929155] R13: 0000000000000067 R14: ffff888126eff640 R15: ffff888126efe840
<4> [211.929168] FS:  0000000000000000(0000) GS:ffff888133d80000(0000) knlGS:0000000000000000
<4> [211.929182] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
<4> [211.929194] CR2: 0000000000000014 CR3: 0000000002610000 CR4: 00000000000006e0
<4> [211.929206] Call Trace:
<4> [211.929294]  i915_read_mch_val+0x15/0x380 [i915]
<4> [211.929309]  ? ips_monitor+0x3fb/0x630 [intel_ips]
<4> [211.929321]  ips_monitor+0x53c/0x630 [intel_ips]
<4> [211.929334]  ? ips_gpu_lower+0x30/0x30 [intel_ips]
<4> [211.929348]  kthread+0x14d/0x170
<4> [211.929358]  ? kthread_park+0x80/0x80
<4> [211.929369]  ret_from_fork+0x22/0x30
<4> [211.929382] Modules linked in: vgem snd_hda_codec_hdmi snd_hda_codec_generic ledtrig_audio i915 coretemp crct10dif_pclmul crc32_pclmul ghash_clmulni_intel snd_hda_intel snd_intel_dspcfg snd_hda_codec snd_hwdep snd_hda_core e1000e snd_pcm mei_me mei intel_ips lpc_ich ptp prime_numbers pps_core
<4> [211.929437] CR2: 0000000000000014
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200915105113.26564-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_rps.c

@@ -1949,7 +1949,7 @@ static struct drm_i915_private *mchdev_get(void)
 
 	rcu_read_lock();
 	i915 = rcu_dereference(ips_mchdev);
-	if (!kref_get_unless_zero(&i915->drm.ref))
+	if (i915 && !kref_get_unless_zero(&i915->drm.ref))
 		i915 = NULL;
 	rcu_read_unlock();