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

[Waiman Long]

On 1/21/22 05:12, Zhang Qiao wrote:
> 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@xxxxxxxxxx>
> Signed-off-by: Zhang Qiao <zhangqiao22@xxxxxxxxxx>
> ---
>   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

The locking sequence looks right.

Acked-by: Waiman Long <longman@xxxxxxxxxx>