Problem when disabled interrupt in system call (ppc8270)

[wilbur.chan]

ppc 8270, kernel 2.6.21.7

I took the following steps:


In a system call function , say , sys_reboot, interrupt was disabled
by local_irq_disable.

Then ,  value at the address of 0xc000050  was set to a value , say ,
0x1234.  Code was like this :

sys_reboot()
{
local_irq_disable();
*(volatile unsigned long * )0xc1000050 = 0x1234;
 while(1)
   {
    ;
   }
}

Finally, I reset the board (with power still on)  into uboot,using
'md 0x1000050' to display the content at physical address 0x1000050,
and found that , it was not 0x1234.


However, if I delete the local_irq_disable() in sys_reboot, everything
went well---After I reset the board, 'md 0x1000050' return the value
0x1234.


So, this really puzzled me , could someone explain why this happed?  Thank you.



PS:

static inline unsigned long local_irq_disable(void)
{
	unsigned long flags, zero;

	__asm__ __volatile__("li %1,0; lbz %0,%2(13); stb %1,%2(13)"
	: "=r" (flags), "=&r" (zero)
	: "i" (offsetof(struct paca_struct, soft_enabled))
	: "memory");

	return flags;
}