From 23b1ab15ca91f2cffaf149d9bb0fafeb700c7db2 Mon Sep 17 00:00:00 2001 From: Waiman Long Date: Fri, 2 Oct 2026 12:54:38 -0400 Subject: cgroup/cpuset: Handle cpu hotplug race in guarantee_active_cpus() With commit 2125c0034c5d ("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 7a0aabd9ce69 ("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: 7a0aabd9ce69 ("cgroup/cpuset: Always use cpu_active_mask") Cc: stable@vger.kernel.org # v6.16+ Reported-by: Farhad Alemi Link: https://lore.kernel.org/lkml/CA+0ovChh3VjsKN1g+ZGjwwY2fGTpP7uD+aCCByLj5Qbymw=bfQ@mail.gmail.com Signed-off-by: Waiman Long Tested-by: Farhad Alemi Signed-off-by: Tejun Heo --- 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 98af44a7f399..d2ec7b128960 100644 --- a/kernel/cgroup/cpuset.c +++ b/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(); } -- cgit v1.2.3