Re: [PATCH] cgroup/cpuset: Handle cpu hotplug race in guarantee_active_cpus()

[Waiman Long]

On 8/30/26 9:50 PM, Ridong Chen wrote:
> On 8/29/2026 2:03 AM, Waiman Long wrote:
> > With commit 2125c0034c5d ("cgroup/cpuset: Make cpuset hotplug
> > processing synchronous"), the cpuset hotplug operation becomes
> > synchronous. That commit also removes the code that handles the race
> > between cpuset_hotplug_work and cpu hotplug notifier with the assumption
> > that race is now gone. Later commit 7a0aabd9ce69 ("cgroup/cpuset:
> > Always use cpu_active_mask") updates the cpuset code to always use
> > cpu_active_mask instead of cpu_ohline_mask in various places including
> > guarantee_online_cpus() which is also renamed to guarantee_active_cpus()
> > in that commit.
> >
> > In the case of CPU offline operation, cpuset_active_mask is updated
> > first in sched_cpu_deactivate() to remove the offline CPU before
> > cpuset_handle_hotplug() is called to update the effective_cpus of the
> > affected cpusets. The cpu_online_mask is updated after that near the end
> > of the offline operation to remove the offline CPU. As a result, the 
> > race
> > comes back and the top cpuset may not have any active CPU leading to 
> > NULL
> > pointer dereference during the race window when guarantee_active_cpus()
> > is called after cpu_active_mask is updated to remove the CPU to be torn
> > down but before cpuset_handle_hotplug() is able to properly update the
> > effective_cpus of the top cpuset.
>
> Hi Longman,
>
> Thank you for the patch. I believe it should resolve the issue.
> Could Farhad please verify whether it works as expected?
>
> > Fix this by adding back the NULL cs check to avoid this problem. 
> > However,
> > the output of guarantee_active_cpus() will be unreliable during that
> > race window.
>
> If guarantee_active_cpus() can return unreliable results during that 
> race window, should it instead return an error when it cannot 
> guarantee that the active CPUs are usable? Otherwise, callers might 
> use an incorrect 'pmask', which could lead to other issues downstream.

I am thinking about that too. However, the change will be more extensive 
and it is that easy to figure out if we are the in the race window. So 
more thought will be needed before we can make that change. In the mean 
time, we have to fix the NULL pointer dereference and it has behaved 
like that before.

Cheers,
Longman