Re: Fw: Re: [mm PATCH 4/6] RCU: (now) CPU hotplug

[Pavel Machek <pavel@ucw.cz>]

Hi!

> > > > 	static int _cpu_down(unsigned int cpu)
> > > > 	{
> > > > 		int err;
> > > > 		struct task_struct *p;
> > > > 		cpumask_t old_allowed, tmp;
> > > > 
> > > > 		if (num_online_cpus() == 1)
> > > > 			return -EBUSY;
> > > > 
> > > > 		if (!cpu_online(cpu))
> > > > 			return -EINVAL;
> > > > 
> > > > 		if (freeze_processes()) {
> > > > 			err = -EBUSY;
> > > > 			goto out_freeze_notify_failed;
> > > > 		}
> > > > 		err = raw_notifier_call_chain(&cpu_chain, CPU_DOWN_PREPARE,
> > > > 							(void *)(long)cpu);
> > > 
> > > yeah. This all looks so nice that i almost cannot believe it's true :-) 
> > 
> > Well, it turns out that maybe it is in fact untrue.  :-/
> > 
> > I need to look at all uses of PF_NOFREEZE -- as I understand the
> > code, processes marked PF_NOFREEZE will continue running, potentially
> > interfering with the hotplug operation.  :-(
> > 
> > I will pass my findings on to this list.
> 
> Well, I did it some time ago, although not very thoroughly.
> 
> AFAICS there are not so many, but one that stands out is the worker threads.
> We needed two of them to actually go to sleep, so now it's possible to create
> a "freezeable workqueue" the worker thread of which will not set PF_NOFREEZE,
> but currently this is only used by XFS.

We should slowly move as workqueues to freezeable ones... Having too
much stuff NOFREEZE is evil, even for swsusp.
								Pavel

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