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Date: Thu, 16 Mar 2017 17:17:57 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Tejun Heo <tj@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>, Chris Mason <clm@fb.com>,
        linux-kernel@vger.kernel.org, kernel-team@fb.com,
        Li Zefan <lizefan@huawei.com>, Johannes Weiner <hannes@cmpxchg.org>,
        cgroups@vger.kernel.org
Subject: Re: [PATCH 2/2] kthread, cgroup: close race window where new
        kthreads can be migrated to non-root cgroups
Message-ID: <20170316161756.GA27613@redhat.com>
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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.