Re: Oops in find_busiest_group(): 2.6.8-rc1-mm1

[Paul Jackson <pj@sgi.com>]

I just hit what might be the same oops.

I had not upgraded my working kernel for a month, and just now, when I
upgraded to 2.6.8-rc2-mm1, running sn2_defconfig on a small SN2 system,
it fails to boot everytime, ending with an Oops that starts out with:

======================================================
Freeing unused kernel memory: 320kB freed
Unable to handle kernel NULL pointer dereference (address 0000000000000008)
swapper[0]: Oops 8813272891392 [1]
Modules linked in:

Pid: 0, CPU 0, comm:              swapper
psr : 0000101008022018 ifs : 8000000000000e20 ip  : [<a0000001000bd710>]    Not tainted
ip is at find_busiest_group+0xb0/0x640
======================================================

I added a conditional printk_ratelimit'ed print at the top of
find_busiest_group() whenever group is NULL, just before the first
dereference of group in the line:

	local_group = cpu_isset(this_cpu, group->cpumask);

That print fires about 20,480 times each 5 second suppression window.

But it boots, if I also add code to break out of the "do { ... } while
(group != sd->groups)" loop, whenever group goes NULL.

-- 
                          I won't rest till it's the best ...
                          Programmer, Linux Scalability
                          Paul Jackson <pj@sgi.com> 1.650.933.1373