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SubjectRe: [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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