Re: [Question] set_cpus_allowed_ptr() call failed at cpuset_attach()

From: Zhang Qiao
Date: Fri Jan 21 2022 - 03:33:05 EST




在 2022/1/20 22:02, Michal Koutný 写道:
> On Thu, Jan 20, 2022 at 03:14:22PM +0800, Zhang Qiao <zhangqiao22@xxxxxxxxxx> wrote:
>> i think the troublesome scenario as follows:
>> cpuset_can_attach
>> down_read(cpuset_rwsem)
>> // check all migratees
>> up_read(cpuset_rwsem)
>> [ _cpu_down / cpuhp_setup_state ]
>> cpuset_attach
>> down_write(cpuset_rwsem)
>> guarantee_online_cpus() // (load cpus_attach)
>> sched_cpu_deactivate
>> set_cpu_active(cpu, false) // 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)
>> schedule_work
>> ...
>> cpuset_hotplug_update_tasks
>> down_write(cpuset_rwsem)
>> up_write(cpuset_rwsem)
>> ... flush_work
>> [ _cpu_down / cpu_up_down_serialize_trainwrecks ]
>
> Thanks, a locking loophole indeed.
>
> FTR, meanwhile I noticed: a) cpuset_fork() looks buggy when
> CLONE_INTO_CGROUP (and dst.cpus != src.cpus), b) it'd be affected with
> similar hotplug race.

Yes, it shouldn't set the current task's cpumak to the child process at cpuset_fork().

Regards,
Qiao
.
>
> Michal
> .
>