Re: cgroup null pointer dereference

[Kamaljit Singh]

Hi Waiman,

>On 4/23/25 1:30 PM, Kamaljit Singh wrote:
>> Hello,
>>
>> While running IOs to an nvme fabrics target we're hitting this null pointer which causes
>> CPU hard lockups and NMI. Before the lockups, the Medusa IOs ran successfully for ~23 hours.
>>
>> I did not find any panics listing nvme or block driver calls.
>>
>> RIP: 0010:cgroup_rstat_flush+0x1d0/0x750
>> points to rstat.c, cgroup_rstat_push_children(), line 162 under second while() to the following code.
>>
>> 160                 /* updated_next is parent cgroup terminated */
>> 161                 while (child != parent) {
>> 162                         child->rstat_flush_next = head;
>> 163                         head = child;
>> 164                         crstatc = cgroup_rstat_cpu(child, cpu);
>> 165                         grandchild = crstatc->updated_children;
>>
>> In my test env I've added a null check to 'child' and re-running the long-term test.
>> I'm wondering if this patch is sufficient to address any underlying issue or is just a band-aid.
>> Please share any known patches or suggestions.
>>               -          while (child != parent) {
>>               +         while (child && child != parent) {
>
>Child can become NULL only if the updated_next list isn't parent
>terminated. This should not happen. A warning is needed if it really
>happens. I will take a further look to see if there is a bug somewhere.

My test re-ran for 36+ hours without any CPU lockups or NMI. This patch seems to have helped.