Re: [PATCH 1/3] srcu: use cpu_online() instead custom check

[Sebastian Andrzej Siewior]

On 2017-09-22 11:43:14 [-0700], Paul E. McKenney wrote:
> On Fri, Sep 22, 2017 at 05:28:04PM +0200, Sebastian Andrzej Siewior wrote:
> > The current check via srcu_online is slightly racy because after looking
> > at srcu_online there could be an interrupt that interrupted us long
> > enough until the CPU we checked against went offline.
> 
> But in that case, wouldn't the interrupt block the synchronize_sched()
> later in the offline sequence?

What I meant is:

  CPU0						CPU1
  preempt_disable();
  if (READ_ONCE(per_cpu(srcu_online, 1)))
  *interrupt*
						WRITE_ONCE(per_cpu(srcu_online, cpu), false);
						and CPU is offnline
  				
	ret = queue_delayed_work_on(1, wq, dwork, delay);

is this possible or are there a safety belt for this?

> More to the point, are you actually seeing this failure, or is this
> a theoretical bug?

I need to get rid of the preempt_disable() section in which
queue_delayed_work*() is invoked for RT.

> > An alternative would be to hold the hotplug rwsem (so the CPUs don't
> > change their state) and then check based on cpu_online() if we queue it
> > on a specific CPU or not. queue_work_on() itself can handle if something
> > is enqueued on an offline CPU but a timer which is enqueued on an offline
> > CPU won't fire until the CPU is back online.
> > 
> > I am not sure if the removal in rcu_init() is okay or not. I assume that
> > SRCU won't enqueue a work item before SRCU is up and ready.
> 
> Another alternative would be to disable preemption across the check and
> the call to queue_delayed_work_on().

you need to ensure the *other* CPU won't in the middle of checking its
status. preempt_disable() won't do this on the other CPU.

> Yet another alternative would be to have an SRCU-specific per-CPU lock
> that is acquired across the setting and clearing of srcu_online,
> and also across the check and the call to queue_delayed_work_on().
> This last would be more consistent with a desire to remove the
> synchronize_sched() from the offline sequence.
> 
> Or am I missing something here?
The perCPU lock should work. And cpus_read_lock() is basically that
except that srcu_online_cpu() is not holding it but the CPU-HP code.

So you want keep things as-is or do you prefer a per-CPU rwsem instead?

Sebastian