linux-kernel - Re: [PATCH] sched/kthread: Complain loudly when others violate our flags

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Date:	Thu, 29 Sep 2011 20:48:15 -0700
From:	Tejun Heo <htejun@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] sched/kthread: Complain loudly when others violate our
 flags

Hello, Steven.

Sorry about the delay, coming back from a rather long vacation.

On Tue, Sep 27, 2011 at 05:17:34PM -0400, Steven Rostedt wrote:
> I looked at the task that it tried to migrate, and it happened to be the
> kworker thread! Then I went into kernel/workqueue.c and found this
> nonsense:
> 
> 	if (bind && !on_unbound_cpu)
> 		kthread_bind(worker->task, gcwq->cpu);
> 	else {
> 		worker->task->flags |= PF_THREAD_BOUND;
> 		if (on_unbound_cpu)
> 			worker->flags |= WORKER_UNBOUND;
> 	}
> 
> Nothing but the scheduler and kthread_bind() has the right to set the
> PF_THREAD_BOUND flag. Especially when the thread IS NOT BOUNDED!!!!!!
> 
> I don't go around and stick my hand down your pants to play with your
> flags! Don't stick your hand in ours and play with our flags!
> 
> WTF is the workqueue code setting the PF_THREAD_BOUND flag manually?
> Talk about fragile coupling! You just made this flag meaningless. Don't
> do that.

IIRC, this was because there was no way to set PF_THREAD_BOUND once a
kthread starts to run and workers can stay active across CPU bring
down/up cycle.  Per-cpu kthreads need PF_THREAD_BOUND to prevent cpu
affinity manipulation by third party for correctness.

> Sorry but I just wasted two whole days because of this nonsense and I'm
> not particularly happy about it.

Sorry that it wasted your time and made you unhappy but wouldn't
grepping for its usage a logical thing if you wanted to add to what it
meant?  PF_THREAD_BOUND meaning the task's affinity or cpuset can't be
manipulated by third party seems like a valid interpretation.

Simply removing it would allow breaking workqueue from userland by
manipulating affinity.  How about testing PF_WQ_WORKER in
set_cpus_allowed_ptr() (and maybe cpuset, I'm not sure)?

Thanks.

-- 
tejun
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