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Date:   Tue, 9 May 2023 12:03:48 +0200
From:   Michal =?utf-8?Q?Koutn=C3=BD?= <mkoutny@suse.com>
To:     Tejun Heo <tj@kernel.org>
Cc:     cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Benjamin Berg <benjamin@sipsolutions.net>
Subject: Re: [RFC PATCH] cgroup: Return error when attempting to migrate a
 zombie process
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On Fri, May 05, 2023 at 09:04:37AM -1000, Tejun Heo <tj@kernel.org> wrote:
> This seems racy to me. The liveness state can change between here and the
> PF_EXITING check in cgroup_migrate_add_task(), right?

You're right, threadgroup lock won't prevent that (as I got wrongly in
the patch):

  cgroup_procs_write_start                                =20
                                                     do_exit
                                                       exit_signals
                                                         cgroup_threadgroup=
_change_begin
                                                           tsk->flags |=3D =
PF_EXITING
                                                         cgroup_threadgroup=
_change_end
    percpu_down_write(&cgroup_threadgroup_rwsem)          =20
      ...                                                 =20
      atomic_read(&live)                                  =20
                                                       ...
                                                       atomic_dec_and_test(=
live)
      ...                                                 =20
      cgroup_migrate_add_task                             =20
      ...                                                 =20
    percpu_up_write(&cgroup_threadgroup_rwsem)


> Wouldn't it be better to just track how many tasks are tracked and
> return -ESRCH if none was migrated?

Thanks, such an integral sounds better, will see.

Michal

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