[PATCH] cgroup/cpuset: Fix a race between cpuset_attach() and cpu hotplug

[Zhang Qiao <zhangqiao22@huawei.com>]

As previously discussed(https://lkml.org/lkml/2022/1/20/51),
cpuset_attach() is affected with similar cpu hotplug race,
as follow scenario:

     cpuset_attach()				cpu hotplug
    ---------------------------            ----------------------
    down_write(cpuset_rwsem)
    guarantee_online_cpus() // (load cpus_attach)
					sched_cpu_deactivate
					  set_cpu_active()
					  // will change cpu_active_mask
    set_cpus_allowed_ptr(cpus_attach)
      __set_cpus_allowed_ptr_locked()
       // (if the intersection of cpus_attach and
         cpu_active_mask is empty, will return -EINVAL)
    up_write(cpuset_rwsem)

To avoid races such as described above, protect cpuset_attach() call
with cpu_hotplug_lock.

Fixes: be367d099270 ("cgroups: let ss->can_attach and ss->attach do whole threadgroups at a time")
Reported-by: Zhao Gongyi <zhaogongyi@huawei.com>
Signed-off-by: Zhang Qiao <zhangqiao22@huawei.com>
---
 kernel/cgroup/cpuset.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index dc653ab26e50..0af5725cc1df 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -2252,6 +2252,7 @@ static void cpuset_attach(struct cgroup_taskset *tset)
 	cgroup_taskset_first(tset, &css);
 	cs = css_cs(css);

+	cpus_read_lock();
 	percpu_down_write(&cpuset_rwsem);

 	guarantee_online_mems(cs, &cpuset_attach_nodemask_to);
@@ -2305,6 +2306,7 @@ static void cpuset_attach(struct cgroup_taskset *tset)
 		wake_up(&cpuset_attach_wq);

 	percpu_up_write(&cpuset_rwsem);
+	cpus_read_unlock();
 }

 /* The various types of files and directories in a cpuset file system */
--
2.18.0