Comment # 44 on bug 1219522 from Takashi Iwai

OK, thanks.  The kernel WARNING with stack trace like the following is no real
crash but it's intentionally showing the stack trace for debugging:
[    8.036410] ------------[ cut here ]------------
[    8.037093] WARNING: CPU: 2 PID: 662 at
drivers/input/mouse/psmouse-base.c:123 psmouse_from_serio+0x1e/0x30

This appears in the both logs.  So far, so good.

Meanwhile, the first log followed another Oops messages:

[    8.094105] RIP: 0010:__mem_cgroup_charge+0xb/0xb0
[    8.095183] Code: 81 58 01 00 00 c3 cc cc cc cc 0f 1f 44 00 00 90 90 90 90
90 90 90 90 90 90 90 90 90 90 90 90 66 0f 1f 00 0f 1f 44 00 00 41 54 <41> 89 d4
55 48 89 fd 48 89 f7 53 e8 35 89 ff ff ba 01 00 00 00 48
[    8.096284] RSP: 0000:ffffb0f2407bbd88 EFLAGS: 00000246
[    8.097449] RAX: 0017ffffc0000000 RBX: ffffb0f2407bbe08 RCX:
0000000000000000
[    8.098584] RDX: 0000000000000cc0 RSI: ffff90d080071600 RDI:
ffffe0e484c8b8c0
[    8.099787] RBP: ffffb0f2407bbe08 R08: ffffe0e484c8b8c0 R09:
ffff90d3ef3404f0
[    8.100868] R10: 0000000000000000 R11: 0000000000000001 R12:
ffff90d082919840
[    8.102187] R13: fffffffffffff000 R14: 0000000000000001 R15:
00007ff171d89000
[    8.103263]  do_anonymous_page+0x23e/0x6e0
[    8.104764]  ? pmdp_invalidate+0x130/0x130
[    8.105930]  __handle_mm_fault+0xb4d/0xe60
[    8.107361]  handle_mm_fault+0x17f/0x360
[    8.108505]  do_user_addr_fault+0x15b/0x670
[    8.109694]  exc_page_fault+0x71/0x160
[    8.110883]  asm_exc_page_fault+0x26/0x30
[    8.112031] RIP: 0033:0x7ff17376c9e4
[    8.113173] Code: 3a e0 c5 f8 77 c3 c5 fe 6f 4e 20 f7 c1 00 0e 00 00 75 65
49 89 c9 48 8d 4c 16 ff 48 83 ce 3f 4a 8d 7c 0e 01 48 29 f1 48 ff c6 <f3> a4 c4
c1 7e 7f 00 c4 c1 7e 7f 48 20 c5 f8 77 c3 66 66 2e 0f 1f
[    8.114373] RSP: 002b:00007ffc29992758 EFLAGS: 00010212
[    8.115565] RAX: 00007ff171d7d010 RBX: 0000555933e399e0 RCX:
0000000000014010
[    8.116710] RDX: 0000000000020000 RSI: 00007ff171e4c000 RDI:
00007ff171d89000
[    8.117976] RBP: 00007ffc29992830 R08: 00007ff171d7d010 R09:
fffffffffff3d000
[    8.119134] R10: 186afaaa2a71579a R11: d9670ae1eee0759f R12:
0000555933e57bda
[    8.120302] R13: 00007ff1735f2be0 R14: 0000000000020000 R15:
00007ff171d7d010
[    8.121613]  </TASK>
[    8.122608] ---[ end trace 0000000000000000 ]---

This is unexpected, and this can be a real problem.  But as it's not visible in
the second log, it might be intermittent.

In anyway, at least the above logs indicate that my guess was correct: it was
the NULL dereference in synaptics driver.

I'm going to submit the fix patch; it might be no best fix, but better than
crash, obviously.