[PATCH] Oops when changing backlight brightness during eeepc-laptop init

[Darren Salt <linux@youmustbejoking.demon.co.uk>]

I got the following oops while changing the backlight brightness during
startup. When it happens, it prevents use of the hotkeys, Fn-Fx, and the lid
button.

It's a clear use-before-init, as I verified by testing with an
appropriately-placed "else printk".

BUG: unable to handle kernel NULL pointer dereference at 00000000
*pde = 00000000 
Oops: 0002 [#1] PREEMPT SMP 
Pid: 160, comm: kacpi_notify Not tainted (2.6.28.1-eee901 #4) 901
EIP: 0060:[<c0264e68>]  [<c0264e68>] eeepc_hotk_notify+26/da
EFLAGS: 00010246 CPU: 1
Using defaults from ksymoops -t elf32-i386 -a i386
EAX: 00000009 EBX: 00000000 ECX: 00000009 EDX: f70dbf64
ESI: 00000029 EDI: f7335188 EBP: c02112c9 ESP: f70dbf80
 DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0068
 f70731e0 f73acd50 c02164ac f7335180 f70aa040 c02112e6 f733518c c012b62f
 f70aa044 f70aa040 c012bdba f70aa04c 00000000 c012be6e 00000000 f70bdf80
 c012e198 f70dbfc4 f70dbfc4 f70aa040 c012bdba 00000000 c012e0c9 c012e091
Call Trace:
 [<c02164ac>] ? acpi_ev_notify_dispatch+4c/55
 [<c02112e6>] ? acpi_os_execute_deferred+1d/25
 [<c012b62f>] ? run_workqueue+71/f1
 [<c012bdba>] ? worker_thread+0/bf
 [<c012be6e>] ? worker_thread+b4/bf
 [<c012e198>] ? autoremove_wake_function+0/2b
 [<c012bdba>] ? worker_thread+0/bf
 [<c012e0c9>] ? kthread+38/5f
 [<c012e091>] ? kthread+0/5f
 [<c0103abf>] ? kernel_thread_helper+7/10
Code: 00 00 00 00 c3 83 3d 60 5c 50 c0 00 56 89 d6 53 0f 84 c4 00 00 00 8d 42
e0 83 f8 0f 77 0f 8b 1d 68 5c 50 c0 89 d8 e8 a9 fa ff ff <89> 03 8b 1d 60 5c
50 c0 89 f2 83 e2 7f 0f b7 4c 53 10 8d 41 01 

Signed-off-by: Darren Salt <linux@youmustbejoking.demon.co.uk>

(The patch is against 2.6.28.1. It's also needed for 2.6.29, in which I see
that the driver is now in drivers/platform/x86. Hardware is a 901 with BIOS
1808.)

--- a/drivers/misc/eeepc-laptop.c
+++ b/drivers/misc/eeepc-laptop.c
@@ -510,7 +510,8 @@
 static void notify_brn(void)
 {
 	struct backlight_device *bd = eeepc_backlight_device;
-	bd->props.brightness = read_brightness(bd);
+	if (bd)
+		bd->props.brightness = read_brightness(bd);
 }
 
 static void eeepc_hotk_notify(acpi_handle handle, u32 event, void *data)

-- 
| Darren Salt    | linux or ds at              | nr. Ashington, | Toon
| RISC OS, Linux | youmustbejoking,demon,co,uk | Northumberland | Army
| + Buy less and make it last longer.         INDUSTRY CAUSES GLOBAL WARMING.

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