Re: [PATCH] rcu/nocb: Extend checks for offloaded rdp by migrate_disable

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2021-07-27 10:23:51 [-0700], Paul E. McKenney wrote:
> > I don't fully understand why the CPU-hotplug lock matters here but this
> > is beside the point ;)
> 
> If I remember correctly, any attempt to change the offloaded state
> must hold off CPU-hotplug operations.  So if the current thread is
> holding off CPU-hotplug operations, no other thread can be doing
> an offload or de-offload operation.

I'm not sure what you mean by "change the offloaded state". If the
CPU-hotplug read-lock is acquired you are still preemptible. So you
could migrate to another CPU at which point this_cpu_ptr(&rcu_data)
would change.

> >  kernel/rcu/tree_plugin.h | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index 0ff5e4fb933e7..d8a623ba7d243 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -57,16 +57,18 @@ static bool rcu_rdp_is_offloaded(struct rcu_data *rdp)
> >  	 * timers have their own means of synchronization against the
> >  	 * offloaded state updaters.
> >  	 */
> > +#ifdef CONFIG_SMP
> >  	RCU_LOCKDEP_WARN(
> >  		!(lockdep_is_held(&rcu_state.barrier_mutex) ||
> >  		  (IS_ENABLED(CONFIG_HOTPLUG_CPU) && lockdep_is_cpus_held()) ||
> >  		  rcu_lockdep_is_held_nocb(rdp) ||
> >  		  (rdp == this_cpu_ptr(&rcu_data) &&
> > -		   !(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible())) ||
> > +		   (!(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible()) ||
> 
> 
> > +		    current->migration_disabled)) ||
> 
> How does this change interact with the one proposed by Valentin?
> 
> https://lore.kernel.org/lkml/20210721115118.729943-3-valentin.schneider@arm.com/

So by looking at the series, it does the same thing. I would prefer
is_pcpu_stable() rather then is_pcpu_safe() but that is a different
topic.
If we settle for this series instead someone should respond to that
thread. Let me see if I find someone.
I'm fine either way :)

Thanks Paul.

> 							Thanx, Paul
> 
> >  		  rcu_current_is_nocb_kthread(rdp)),
> >  		"Unsafe read of RCU_NOCB offloaded state"
> >  	);
> > -
> > +#endif
> >  	return rcu_segcblist_is_offloaded(&rdp->cblist);
> >  }
> >  

Sebastian