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Date: Thu, 16 Mar 2017 12:07:34 -0400
From: Tejun Heo <tj@kernel.org>
To: Oleg Nesterov <oleg@redhat.com>
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
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Hello,

On Thu, Mar 16, 2017 at 04:39:26PM +0100, Oleg Nesterov wrote:
> Either way, I am wondering if we can do something really trivial like
> the patch below. This way we can also remove the "tsk == kthreadd_task"
> check, and we do not need the barriers.
> 
> Oleg.
> 
> --- x/kernel/kthread.c
> +++ x/kernel/kthread.c
> @@ -226,6 +226,7 @@
>  	ret = -EINTR;
>  	if (!test_bit(KTHREAD_SHOULD_STOP, &self->flags)) {
>  		__kthread_parkme(self);
> +		current->flags &= ~PF_IDONTLIKECGROUPS;
>  		ret = threadfn(data);
>  	}
>  	do_exit(ret);
> @@ -537,7 +538,7 @@
>  	set_cpus_allowed_ptr(tsk, cpu_all_mask);
>  	set_mems_allowed(node_states[N_MEMORY]);
>  
> -	current->flags |= PF_NOFREEZE;
> +	current->flags |= (PF_NOFREEZE | PF_IDONTLIKECGROUPS);
>  
>  	for (;;) {
>  		set_current_state(TASK_INTERRUPTIBLE);
> --- x/kernel/cgroup/cgroup.c
> +++ x/kernel/cgroup/cgroup.c
> @@ -2429,7 +2429,7 @@
>  	 * trapped in a cpuset, or RT worker may be born in a cgroup
>  	 * with no rt_runtime allocated.  Just say no.
>  	 */
> -	if (tsk == kthreadd_task || (tsk->flags & PF_NO_SETAFFINITY)) {
> +	if (tsk->flags & (PF_NO_SETAFFINITY | PF_IDONTLIKECGROUPS)) {
>  		ret = -EINVAL;
>  		goto out_unlock_rcu;
>  	}

Absolutely.  If we're willing to spend a PF flag on it, we can
properly wait for it too instead of failing it.

Thanks.

-- 
tejun