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

[Waiman Long <longman@redhat.com>]

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.

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.

Fixes: 7a0aabd9ce69 ("cgroup/cpuset: Always use cpu_active_mask")
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>
---
 kernel/cgroup/cpuset.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 0bad5bbec6bf..309478ab21ad 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -513,10 +513,26 @@ 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();
 }
 
-- 
2.55.0