Re: [PATCH 2/2] kthread, cgroup: close race window where new kthreads can be migrated to non-root cgroups

From	Oleg Nesterov <oleg@redhat.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH 2/2] kthread, cgroup: close race window where new kthreads can be migrated to non-root cgroups
Date	2017-03-16 17:30 +0100
Message-ID	<tlGrL-X3-1@gated-at.bofh.it> (permalink)
References	<tlqmZ-6ih-3@gated-at.bofh.it> <tlqmZ-6ih-1@gated-at.bofh.it> <tlFvH-gv-9@gated-at.bofh.it> <tlG8q-LZ-15@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On 03/16, Tejun Heo wrote:
>
> Hello,
>
> On Thu, Mar 16, 2017 at 04:02:34PM +0100, Oleg Nesterov wrote:
> > > +bool kthread_initialized(struct task_struct *k)
> > > +{
> > > +	struct kthread *kthread = to_kthread(k);
> > > +
> > > +	return kthread && test_bit(KTHREAD_INITIALIZED, &kthread->flags);
> > > +}
> >
> > Not sure I understand...
> >
> > With this patch you can no longer migrate a kernel thread created by
> > kernel_thread() ? Note that to_kthread() is NULL unless it was created
> > by kthread_create().
>
> Yeah, what it does is preventing migration of kthreads until the
> kthread owner wakes it up for the first time.  The problem is that
> kthread_bind() seals up future cgroup migrations from userland but
> doesn't move back the kthread to the root cgroup, so the userland has
> a window where it can mangle with cgroup membership inbetween and
> break things.

This is clear,

> The NULL test is there because the test may be performed before the
> kthread itself sets up its struct kthread.

This too.

But this also means that __cgroup_procs_write() will always fail if
this task is a kernel thread which was not created by kthread_create().

Currently you can use kernel_thread() (although you shouldn't) and it
can be migrated, this won't work after your patch.

Oleg.

Thread

[PATCH 2/2] kthread, cgroup: close race window where new kthreads  can be migrated to non-root cgroups Tejun Heo <tj@kernel.org> - 2017-03-16 00:20 +0100
  Re: [PATCH 2/2] kthread, cgroup: close race window where new         kthreads can be migrated to non-root cgroups Oleg Nesterov <oleg@redhat.com> - 2017-03-16 16:30 +0100
    Re: [PATCH 2/2] kthread, cgroup: close race window where new         kthreads can be migrated to non-root cgroups Oleg Nesterov <oleg@redhat.com> - 2017-03-16 16:50 +0100
      Re: [PATCH 2/2] kthread, cgroup: close race window where new  kthreads can be migrated to non-root cgroups Tejun Heo <tj@kernel.org> - 2017-03-16 17:10 +0100
        Re: [PATCH 2/2] kthread, cgroup: close race window where new         kthreads can be migrated to non-root cgroups Oleg Nesterov <oleg@redhat.com> - 2017-03-16 17:40 +0100
          Re: [PATCH 2/2] kthread, cgroup: close race window where new  kthreads can be migrated to non-root cgroups Tejun Heo <tj@kernel.org> - 2017-03-16 18:50 +0100
    Re: [PATCH 2/2] kthread, cgroup: close race window where new  kthreads can be migrated to non-root cgroups Tejun Heo <tj@kernel.org> - 2017-03-16 17:10 +0100
      Re: [PATCH 2/2] kthread, cgroup: close race window where new         kthreads can be migrated to non-root cgroups Oleg Nesterov <oleg@redhat.com> - 2017-03-16 17:30 +0100
        Re: [PATCH 2/2] kthread, cgroup: close race window where new  kthreads can be migrated to non-root cgroups Tejun Heo <tj@kernel.org> - 2017-03-16 18:20 +0100
  [PATCH v2] cgroup, kthread: close race window where new kthreads can  be migrated to non-root cgroups Tejun Heo <tj@kernel.org> - 2017-03-16 22:10 +0100
    Re: [PATCH v2] cgroup, kthread: close race window where new         kthreads can be migrated to non-root cgroups Oleg Nesterov <oleg@redhat.com> - 2017-03-17 15:00 +0100
      Re: [PATCH v2] cgroup, kthread: close race window where new kthreads  can be migrated to non-root cgroups Tejun Heo <tj@kernel.org> - 2017-03-17 15:50 +0100

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