On Tue, Jul 27, 2021 at 09:26:05PM +0200, Sebastian Andrzej Siewior wrote: > 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. Sure but it's fine to read the offloaded state of a remote CPU if CPU hotplug lock is held. > > > 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@xxxxxxx/ > > 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 :) Either way I believe it won't work, please check: https://lore.kernel.org/lkml/20210727230814.GC283787@lothringen/ I wonder also, how many similar assumption out there do we have that rely on softirqs not being preemptible. Most of them probably silent. Thanks.
