Re: 2.6.8.1-mm1

[Nathan Lynch <nathanl@austin.ibm.com>]

I can consistently hit this BUG_ON in migration_call's CPU_DEAD handling
by doing:

while true ; do
echo 0 > /sys/devices/system/cpu/cpu1/online
echo 1 > /sys/devices/system/cpu/cpu1/online
done

and then starting a kernel build.  It seems to take less than 20
minutes.  I can also recreate it on ppc64, but only with
CONFIG_PREEMPT=y (I haven't tried without preempt on i386 yet).

kernel BUG at kernel/sched.c:4035!                                              
invalid operand: 0000 [#1]                                                      
PREEMPT SMP                                                                     
Modules linked in:                                                              
CPU:    0                                                                       
EIP:    0060:[<c0122b1b>]    Not tainted VLI                                    
EFLAGS: 00010202   (2.6.8.1-mm1)                                                
EIP is at migration_call+0x1bb/0x300                                            
eax: 00000001   ebx: c1410f60   ecx: 00000000   edx: deee8ec8                   
esi: deee8000   edi: 00000001   ebp: deee8ee0   esp: deee8eb8                   
ds: 007b   es: 007b   ss: 0068                                                  
Process bash (pid: 2956, threadinfo=deee8000 task=df448150)                     
Stack: deee8ed4 de6f6630 de6f6630 deee8ed4 00000296 deee8ee4 00000296 c04a5b78  
       00000001 00000006 deee8ef4 c0136928 deee8000 00000001 c04a7a40 deee8f2c  
       c01405ac c01a8d79 c04f5700 00000000 deee8000 00000001 00000001 000000ff  
Call Trace:                                                                     
 [<c01082ea>] show_stack+0x7a/0x90                                              
 [<c0108472>] show_registers+0x152/0x1c0                                        
 [<c01086a0>] die+0x110/0x200                                                   
 [<c0108b99>] do_invalid_op+0xe9/0xf0                                           
 [<c0107ed9>] error_code+0x2d/0x38                                              
 [<c0136928>] notifier_call_chain+0x28/0x50                                     
 [<c01405ac>] cpu_down+0x11c/0x230                                              
 [<c02a5d98>] store_online+0x38/0x40                                            
 [<c02a3117>] sysdev_store+0x37/0x40                                            
 [<c01a8b8e>] flush_write_buffer+0x2e/0x40                                      
 [<c01a8bea>] sysfs_write_file+0x4a/0x60                                        
 [<c016b362>] vfs_write+0xa2/0x100                                              
 [<c016b471>] sys_write+0x41/0x70                                               
 [<c010746f>] syscall_call+0x7/0xb                                              
Code: 0b cd 0f 6f 54 43 c0 eb 8e c7 04 24 6f 34 43 c0 b8 54 2a 12 c0 89 44 24 04
 e8 e2 3f 00 00 0f 0b 89 00 82 d4 42 c0 e9 4a ff ff ff <0f> 0b c3 0f 6f 54 43 c0
 e9 2c ff ff ff e8 53 87 2f 00 e9 17 ff

Nathan