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Date:   Mon, 19 Sep 2022 12:51:49 +0200
From:   Frederic Weisbecker <frederic@kernel.org>
To:     Pingfan Liu <kernelfans@gmail.com>
Cc:     rcu@vger.kernel.org, "Paul E. McKenney" <paulmck@kernel.org>,
        David Woodhouse <dwmw@amazon.co.uk>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        "Jason A. Donenfeld" <Jason@zx2c4.com>
Subject: Re: [PATCHv2 1/3] rcu: Keep qsmaskinitnext fresh when
 rcutree_online_cpu()
Message-ID: <20220919105149.GA58144@lothringen>
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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@kernel.org> 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.