Re: [PATCH -rt 5/5] cpu-hotplug: cpu_down vs preempt-rt

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Jun 10, 2008 at 05:51:18PM +0200, Peter Zijlstra wrote:
> On Tue, 2008-06-10 at 08:33 -0700, Paul E. McKenney wrote:
> > On Tue, Jun 10, 2008 at 01:13:04PM +0200, Peter Zijlstra wrote:
> > > idle_task_exit() calls mmdrop() from the idle thread, but in PREEMPT_RT all the
> > > allocator locks are sleeping locks - for obvious reasons scheduling away the
> > > idle thread gives some curious problems.
> > > 
> > > Solve this by pushing the mmdrop() into an RCU callback, however we can't use
> > > RCU because the CPU is already down and all the local RCU state has been
> > > destroyed.
> > > 
> > > Therefore create a new call_rcu() variant that enqueues the callback on an
> > > online cpu.
> > 
> > I am a bit nervous about the non-determinism, but on the other hand
> > CPU online/offline events can only happen so often due to the locking.
> > 
> > So...
> > 
> > Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> Thanks!
> 
> Yesterday you suggested using rcu_cpu_online_map and fliplock to avoid
> the loop here:
> 
> > > +void fastcall call_rcu_preempt_online(struct rcu_head *head,
> > > +		void (*func)(struct rcu_head *rcu))
> > > +{
> > > +	struct rcu_data *rdp;
> > > +	unsigned long flags;
> > > +	int cpu;
> > > +
> > > +	head->func = func;
> > > +	head->next = NULL;
> > > +again:
> > > +	cpu = first_cpu(cpu_online_map);
> > > +	rdp = RCU_DATA_CPU(cpu);
> > > +
> > > +	spin_lock_irqsave(&rdp->lock, flags);
> > > +	if (unlikely(!cpu_online(cpu))) {
> > > +		/*
> > > +		 * cpu is removed from the online map before rcu_offline_cpu
> > > +		 * is called.
> > > +		 */
> > > +		spin_unlock_irqrestore(&rdp->lock, flags);
> > > +		goto again;
> > > +	}
> > > +
> > > +	*rdp->nexttail = head;
> > > +	rdp->nexttail = &head->next;
> > > +	spin_unlock_irqrestore(&rdp->lock, flags);
> > > +
> > > +}
> 
> But then the code would look like:
> 
>   spin_lock_irqsave(&rcu_ctrlblk.fliplock, flags);
>   cpu = first_cpu(rcu_cpu_online_map);
>   rdp = RCU_DATA_CPU(cpu);
>   spin_lock(&rdp->lock);
> 
> creating a nesting between these two locks, where I could not find one.
> 
> Do you still prefer I look into changing it into such a form, or are you
> sufficiently non-caring that the current code can stand? :-)

I am equally bothered by the non-determinism and by the nesting, hence
the current code can stand, at least until it causes a real problem.

							Thanx, Paul