Re: [BUG] cpu-hotplug: Can't offline the CPU with naughty realtime processes

[Satoru Takeuchi]

At Mon, 07 May 2007 23:42:53 +1000,
Rusty Russell wrote:
> 
> On Mon, 2007-05-07 at 19:10 +0900, Satoru Takeuchi wrote:
> > Hi,
> > 
> > I found a bug on 2.6.21 cpu-hotplug code.
> > 
> > When process A on CPU0 try to offline the CPU1 on which the process B,
> > realtime process (its task->policy == SCHED_FIFO or SCHED_RR) running
> > without sleep or yield, both CPU0 and CPU1 get hang. It's because of
> > the following code on __stop_machine_run().
> > 
> > struct task_struct *__stop_machine_run(int (*fn)(void *), void *data,
> > 				       unsigned int cpu)
> > {
> > 	...
> > 	p = kthread_create(do_stop, &smdata, "kstopmachine");
> > 	if (!IS_ERR(p)) {
> > 		kthread_bind(p, cpu);
> > 		wake_up_process(p);
> > 		wait_for_completion(&smdata.done);
> > 	}
> > 	...
> > }
> > 
> > kstopmachine is created, bound to the CPU1, and woken up here, but
> > this process can't start to run because reschedule doesn't occur on
> > CPU1. Hence CPU0 also be able to run because it's waiting completion
> > of CPU1's offline work.
> 
> Yes, we should probably move the set_scheduler call in stop_machine
> (where the thread up-prioritizes itself) to before wake_up_process(p),
> to avoid this happening.
> 
> Others have suggested we use the freezer; I've always distrusted that
> code.  It's much trickier than stop_machine().
> 
> I look forward to your patch!
> Rusty.

Thanks, I'll do. Maybe this work will take several days including test.

BTW, how should I manage rt process having max priority as Gautham said?
He said that it's OK unless such kernel thread exists. However, currently
MAX_USER_RT_PRIORITY is equal to MAX_RT_PRIO, so user process also be able
to cause this problem. Is Srivatsa's idea 2 acceptable? Or just apply
"Shouldn't abuse highest rt proority" rule?

Thanks,

Satoru
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