On Mon, Sep 19, 2022 at 06:24:21PM +0800, Pingfan Liu wrote:
> On Fri, Sep 16, 2022 at 10:52 PM Frederic Weisbecker
> <frederic@xxxxxxxxxx> wrote:
> > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > > index 438ecae6bd7e..ef6d3ae239b9 100644
> > > --- a/kernel/rcu/tree_plugin.h
> > > +++ b/kernel/rcu/tree_plugin.h
> > > @@ -1224,7 +1224,7 @@ static void rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
> > > static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
> > > {
> > > struct task_struct *t = rnp->boost_kthread_task;
> > > - unsigned long mask = rcu_rnp_online_cpus(rnp);
> > > + unsigned long mask;
> > > cpumask_var_t cm;
> > > int cpu;
> > >
> > > @@ -1233,6 +1233,11 @@ static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
> > > if (!zalloc_cpumask_var(&cm, GFP_KERNEL))
> > > return;
> > > mutex_lock(&rnp->boost_kthread_mutex);
> > > + /*
> > > + * Relying on the lock to serialize, so when onlining, the latest
> > > + * qsmaskinitnext is for cpus_ptr.
> > > + */
> > > + mask = rcu_rnp_online_cpus(rnp);
> > > for_each_leaf_node_possible_cpu(rnp, cpu)
> > > if ((mask & leaf_node_cpu_bit(rnp, cpu)) &&
> > > cpu != outgoingcpu)
> >
> > Right but you still race against concurrent rcu_report_dead() doing:
> >
> > WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext & ~mask)
> >
>
> Ah. Indeed, my commit log is not precisely described.
>
> In fact, either speedup smp_init [1] or fast kexec reboot [2] still
> uses the model: one hotplug initiator and several reactors. The
> initiators are still excluded from each other by the pair
> cpu_maps_update_begin()/cpu_maps_update_done(). So there can not be a
> cpu-up and cpu-down event at the same time.
Yes but two downing CPUs may race right?
So isn't it possible to have rcu_report_dead() racing against
rcutree_offline_cpu()?
Thanks.
