cgroup/cpuset: Handle cpu hotplug race in guarantee_active_cpus()

With commit 2125c0034c ("cgroup/cpuset: Make cpuset hotplug processing
synchronous"), the cpuset hotplug operation becomes synchronous. That
commit also removes the code that handles race between cpuset_hotplug_work
and cpu hotplug notifier with the assumption that race is now gone.
Later commit 7a0aabd9ce ("cgroup/cpuset: Always use cpu_active_mask")
updates the cpuset code to always use cpu_active_mask instead of
cpu_online_mask in some places including guarantee_online_cpus() which
is renamed to guarantee_active_cpus() in that commit.

In the case of CPU offline operation, cpu_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.

Fix this by adding back the NULL cs check to avoid this problem.

Fixes: 7a0aabd9ce ("cgroup/cpuset: Always use cpu_active_mask")
Cc: stable@vger.kernel.org # v6.16+
Reported-by: Farhad Alemi <farhad.alemi@berkeley.edu>
Link: https://lore.kernel.org/lkml/CA+0ovChh3VjsKN1g+ZGjwwY2fGTpP7uD+aCCByLj5Qbymw=bfQ@mail.gmail.com
Signed-off-by: Waiman Long <longman@redhat.com>
Tested-by: Farhad Alemi <farhad.alemi@berkeley.edu>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup/cpuset.c

@@ -518,10 +518,26 @@ static void guarantee_active_cpus(struct task_struct *tsk,
 	rcu_read_lock();
 	cs = task_cs(tsk);
 
-	while (!cpumask_intersects(cs->effective_cpus, pmask))
+	while (!cpumask_intersects(cs->effective_cpus, pmask)) {
 		cs = parent_cs(cs);
-
+		if (unlikely(!cs)) {
+			/*
+			 * The top cpuset doesn't have any active cpu as a
+			 * consequence of a race between its caller and the cpu
+			 * hotplug operation where cpu_active_mask is updated
+			 * asynchronously before cpuset_handle_hotplug() is
+			 * being called to adjust the effective_cpus of the
+			 * affected cpusets. But we know the top cpuset's
+			 * effective_cpus is on its way to be identical to
+			 * cpu_active_mask minus the exclusive CPUs dedicated
+			 * to other valid cpuset partitions. Just pass back
+			 * the filtered cpu_active_mask in this case.
+			 */
+			goto out_unlock;
+		}
+	}
 	cpumask_and(pmask, pmask, cs->effective_cpus);
+out_unlock:
 	rcu_read_unlock();
 }