cgroup null pointer dereference

[Waiman Long]

On 4/24/25 8:53 PM, Kamaljit Singh wrote:
> 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.
>
I now see what is wrong. The cgroup_rstat_push_children() function is 
supposed to be called with cgroup_rstat_lock held, but commit 
093c8812de2d3 ("cgroup: rstat: Cleanup flushing functions and locking") 
changes that. Hence racing can corrupt the list. I will work on a patch 
to fix that regression.

Cheers,
Longman