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@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 :) 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
