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Date:   Thu, 20 Jan 2022 15:02:22 +0100
From:   Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>
To:     Zhang Qiao <zhangqiao22@huawei.com>
Cc:     Tejun Heo <tj@kernel.org>, lizefan.x@bytedance.com,
        hannes@cmpxchg.org, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [Question] set_cpus_allowed_ptr() call failed at cpuset_attach()
Message-ID: <20220120140222.GA27269@blackbody.suse.cz>
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On Thu, Jan 20, 2022 at 03:14:22PM +0800, Zhang Qiao <zhangqiao22@huawei.com> 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.

Michal