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SubjectRe: [crash] BUG: unable to handle kernel NULL pointer dereference at (null), last sysfs file: /sys/devices/pci0000:00/0000:00:01.0/local_cpus

On Fri, 18 Sep 2009, Ingo Molnar wrote:
> [ 158.058140] warning: `dbus-daemon' uses 32-bit capabilities (legacy support in use)
> [ 159.370562] BUG: unable to handle kernel NULL pointer dereference at (null)
> [ 159.372694] IP: [<ffffffff8143b722>] bitmap_scnprintf+0x72/0xd0

Hmm. The code is

a: 49 63 fc movslq %r12d,%rdi
d: 0f 49 d3 cmovns %ebx,%edx
10: c1 f8 1f sar $0x1f,%eax
13: 4c 01 ff add %r15,%rdi
16: c1 e8 1a shr $0x1a,%eax
19: c1 fa 06 sar $0x6,%edx
1c: 41 c1 e8 02 shr $0x2,%r8d
20: 8d 0c 03 lea (%rbx,%rax,1),%ecx
23: 48 63 d2 movslq %edx,%rdx
26: 83 e1 3f and $0x3f,%ecx
29: 29 c1 sub %eax,%ecx
2b:* 49 8b 44 d5 00 mov 0x0(%r13,%rdx,8),%rax <-- trapping instruction
30: 48 c7 c2 8c 37 16 82 mov $0xffffffff8216378c,%rdx
37: 48 d3 e8 shr %cl,%rax
3a: 89 f1 mov %esi,%ecx
3c: 44 89 f6 mov %r14d,%esi

and the obvious reason seems to be that 'maskp' is NULL (that faulting
thing is the code for "val = (maskp[word] >> bit) & chunkmask;" with the
actual fault being the access of "maskp[word]".

Now, the caller does

mask = cpumask_of_pcibus(to_pci_dev(dev)->bus);

and then uses cpumask_scnprintf() that is just a wrapper that does

bitmap_scnprintf(buf, len, cpumask_bits(srcp), nr_cpumask_bits);

So clearly we have "cpumask_of_pcibus()" being NULL (cpumask_bits() would
not change it).

I assume this is the NUMA case? The non-NUMA case has just

static inline const struct cpumask *cpumask_of_node(int node)
return cpu_online_mask;

so I don't think you can ever get NULL (if we have a NULL cpu_online_mask
we have bigger problems).

[ Side note: looking closer, I think our headers are buggy, and I _know_
they are confusing. The above inline declaration of cpumask_of_node()
seems to be then later overridden in <asm-generic/topology.h> by a

And if I read that right, that will also override the debugging
versions that we declared if CONFIG_DEBUG_PER_CPU_MAPS is on. Ingo?
Rusty? Am I missing something?

That said, those overrides should only happen for non-NUMA ]

The NUMA version of 'cpumask_of_node()' has all the debug code for show
it's not returning NULL, but only when CONFIG_DEBUG_PER_CPU_MAPS is
enabled. Otherwise it all seems to boil down to (through cpumask_of_pcibus
and __pcibus_to_node):


and it can fail either because "node" isn't initialized, or
node_to_cpumask_map[] isn't.

Probably 'node' is still -1, and it gets the NULL by going off the array
into la-la-land.


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