Re: [PATCH 4.19 01/34] cgroup/cpuset: Fix a race between cpuset_attach() and cpu hotplug

Greg Kroah-Hartman
2022-03-14T08:07:51Z

On Tue, Mar 08, 2022 at 04:12:32PM +0100, Michal Koutný wrote:
> Hello.
>
> On Mon, Feb 28, 2022 at 06:24:07PM +0100, Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote:
> > [...]
> > 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)
> > [...]
> > --- a/kernel/cgroup/cpuset.c
> > +++ b/kernel/cgroup/cpuset.c
> > @@ -1528,6 +1528,7 @@ static void cpuset_attach(struct cgroup_
> > cgroup_taskset_first(tset, &css);
> > cs = css_cs(css);
> >
> > + cpus_read_lock();
> > mutex_lock(&cpuset_mutex);
>
> This backport (and possible older kernels) looks suspicious since it comes
> before commit d74b27d63a8b ("cgroup/cpuset: Change cpuset_rwsem and
> hotplug lock order") v5.4-rc1~176^2~30 when the locking order was:
> cpuset lock, cpus lock.
>
> At the same time it also comes before commit 710da3c8ea7d ("sched/core:
> Prevent race condition between cpuset and __sched_setscheduler()")
> v5.4-rc1~176^2~27 when neither __sched_setscheduler() cared and this
> race is similar. (The swapped locking may still conflict with
> rebuild_sched_domains() before d74b27d63a8b.)

Thanks for noticing this. What do you recommend to do to resolve this?

thanks,

greg k-h