cpu hotplug broken in 2.6.9-rc4

[Nathan Lynch]

(Urgh, sent this to the wrong list initially, sorry.  Second try...)

Hi-

Seems that cpu hotplug got broken when benh's monster cleanup patch went
into bk (in 2.6.9-rc2-bk10).  System boots fine, but if I take down a
cpu and then try to bring it back up, I get:

# echo 1 > /sys/devices/system/cpu/cpu1/online 
Bad kernel stack pointer 7d23080 at 6373c0
cpu 0x1: Vector: c000000002ff4d80  at [c0000000077c7d40]
    pc: 00000000006373c0
    lr: 00000000006373c0
    sp: 7d23080
   msr: 1002
  current = 0xc00000000779a8a0
  paca    = 0xc000000000493d00
    pid   = 0, comm = swapper
enter ? for help
1:mon> t
SP (7d23080) is in userspace
1:mon> r
R00 = 0000000000000000   R16 = 0000000000000000
R01 = 0000000007d23080   R17 = 0000000000000000
R02 = 0000000007ad4b68   R18 = 0000000000000000
R03 = 0000000000000001   R19 = 0000000000000000
R04 = 00000000006373c0   R20 = c000000000493880
R05 = 0000000000000001   R21 = 00016bb01a585f1a
R06 = 0000000000000020   R22 = c000000000493d00
R07 = fffffffd00000000   R23 = c000000002565008
R08 = 00000000000d6000   R24 = 0000000000000001
R09 = c00000000737ef80   R25 = 0000000000000000
R10 = c00000000737ed40   R26 = 0000000000000008
R11 = c00000000737eb40   R27 = 0000000000000010
R12 = 0000000000000001   R28 = c0000000077a4000
R13 = c000000000493d00   R29 = 0000000007889698
R14 = 0000000000000000   R30 = c0000000077a4010
R15 = 0000000007ab0420   R31 = 0000000007d23080
pc  = 00000000006373c0
lr  = 00000000006373c0
msr = 0000000000001002   cr  = 22000024
ctr = 800000000010dd60   xer = 0000000000000001   trap = c000000002ff4d80

For what it's worth, the least significant half of pc (00000000006373c0)
matches the address of pseries_secondary_smp_init in the System.map:

c0000000006373c0 D pseries_secondary_smp_init

If I revert the monster patch from the 2.6.9-rc2-bk10 snapshot things
work fine.  I haven't been able to figure out yet how the stack pointer
gets a bad value.

Nathan