Re: [PATCH] rcu-tasks: Make rude RCU-Tasks work well with CPU hotplug

[Joel Fernandes]

On Sat, Nov 26, 2022 at 02:43:59AM +0000, Zhang, Qiang1 wrote:
> On Fri, Nov 25, 2022 at 11:54:27PM +0800, Zqiang wrote:
> > Currently, for the case of num_online_cpus() <= 1, return directly,
> > indicates the end of current grace period and then release old data.
> > it's not accurate, for SMP system, when num_online_cpus() is equal
> > one, maybe another cpu that in offline process(after invoke
> > __cpu_disable()) is still in the rude RCU-Tasks critical section
> > holding the old data, this lead to memory corruption.
> > 
> > Therefore, this commit add cpus_read_lock/unlock() before executing
> > num_online_cpus().
> 
> 
> >I am not sure if this is needed. The only way what you suggest can happen is
> >if the tasks-RCU protected data is accessed after the num_online_cpus() value is
> >decremented on the CPU going offline.
> >
> >However, the number of online CPUs value is changed on a CPU other than the
> >CPU going offline.
> >
> >So there's no way the CPU going offline can run any code (it is already
> >dead courtesy of CPUHP_AP_IDLE_DEAD). So a corruption is impossible.
> >
> >Or, did I miss something?
> 
> Hi joel
> 
> Suppose the system has two cpus
> 
> 	CPU0                                                                     CPU1
> 					     cpu_stopper_thread
>                                                                                   take_cpu_down
> 						    __cpu_disable
> 							dec __num_online_cpus 
>  rcu_tasks_rude_wait_gp                                      cpuhp_invoke_callback

Thanks for clarifying!

You are right, this can be a problem for anything in the stop machine on the
CPU going offline from CPUHP_AP_ONLINE to CPUHP_AP_IDLE_DEAD, during which
the code execute on that CPU is not accounted for in num_online_cpus().

Actually Neeraj found a similar issue 2 years ago and instead of hotplug
lock, he added a new attribute to rcu_state to track number of CPUs.

See:
https://lore.kernel.org/r/20200923210313.GS29330@paulmck-ThinkPad-P72
https://www.mail-archive.com/linux-kernel@xxxxxxxxxxxxxxx/msg2317853.html

Could we do something similar?

Off note is the comment in that thread:
  Actually blocking CPU hotplug would not only result in excessive overhead,
  but would also unnecessarily impede CPU-hotplug operations.

Neeraj is also on the thread and could chime in.

Thanks,

 - Joel


> 	num_online_cpus() == 1
> 		return;
>         
> when __num_online_cpus == 1, the CPU1 not completely offline.
> 
> Thanks
> Zqiang
> 
> >
> >thanks,
> >
> > - Joel
> 
> 
> 
> > 
> > Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
> > ---
> >  kernel/rcu/tasks.h | 20 ++++++++++++++++++--
> >  1 file changed, 18 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> > index 4a991311be9b..08e72c6462d8 100644
> > --- a/kernel/rcu/tasks.h
> > +++ b/kernel/rcu/tasks.h
> > @@ -1033,14 +1033,30 @@ static void rcu_tasks_be_rude(struct work_struct *work)
> >  {
> >  }
> >  
> > +static DEFINE_PER_CPU(struct work_struct, rude_work);
> > +
> >  // Wait for one rude RCU-tasks grace period.
> >  static void rcu_tasks_rude_wait_gp(struct rcu_tasks *rtp)
> >  {
> > +	int cpu;
> > +	struct work_struct *work;
> > +
> > +	cpus_read_lock();
> >  	if (num_online_cpus() <= 1)
> > -		return;	// Fastpath for only one CPU.
> > +		goto end;// Fastpath for only one CPU.
> >  
> >  	rtp->n_ipis += cpumask_weight(cpu_online_mask);
> > -	schedule_on_each_cpu(rcu_tasks_be_rude);
> > +	for_each_online_cpu(cpu) {
> > +		work = per_cpu_ptr(&rude_work, cpu);
> > +		INIT_WORK(work, rcu_tasks_be_rude);
> > +		schedule_work_on(cpu, work);
> > +	}
> > +
> > +	for_each_online_cpu(cpu)
> > +		flush_work(per_cpu_ptr(&rude_work, cpu));
> > +
> > +end:
> > +	cpus_read_unlock();
> >  }
> >  
> >  void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t func);
> > -- 
> > 2.25.1
> >