Re: [PATCH RFC] v7 expedited "big hammer" RCU grace periods

Paul E. McKenney
2009-05-26T15:46:42Z

On Mon, May 25, 2009 at 06:28:43PM -0700, Paul E. McKenney wrote:
> On Tue, May 26, 2009 at 09:03:55AM +0800, Lai Jiangshan wrote:
> > Paul E. McKenney wrote:
> > >
> > > Good point -- I should at the very least add a comment to
> > > synchronize_sched_expedited() stating that it cannot be called holding
> > > any lock that is acquired in a CPU hotplug notifier. If this restriction
> > > causes any problems, then your approach seems like a promising fix.
> >
> > Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
>
> Thank you very much for your review and comments!!!
>
> > >> The coupling of synchronize_sched_expedited() and migration_req
> > >> is largely increased:
> > >>
> > >> 1) The offline cpu's per_cpu(rcu_migration_req, cpu) is handled.
> > >> See migration_call::CPU_DEAD
> > >
> > > Good. ;-)
> > >
> > >> 2) migration_call() is the highest priority of cpu notifiers,
> > >> So even any other cpu notifier calls synchronize_sched_expedited(),
> > >> It'll not cause DEADLOCK.
> > >
> > > You mean if using your preempt_disable() approach, right? Unless I am
> > > missing something, the current get_online_cpus() approach would deadlock
> > > in this case.
> >
> > Yes, I mean if using my preempt_disable() approach. The current
> > get_online_cpus() approach would NOT deadlock in this case also,
> > we can require get_online_cpus() in cpu notifiers.
>
> I have added the comment for the time being, but should people need to
> use this in CPU-hotplug notifiers, then again your preempt_disable()
> approach looks to be a promising fix.

I looked more closely at your preempt_disable() suggestion, which you
presented earlier as follows:

> I think we can reuse req->dest_cpu and remove get_online_cpus().
> (and use preempt_disable() and for_each_possible_cpu())
>
> req->dest_cpu = -2 means @req is not queued
> req->dest_cpu = -1 means @req is queued
>
> a little like this code:
>
> mutex_lock(&rcu_sched_expedited_mutex);
> for_each_possible_cpu(cpu) {
> preempt_disable()
> if (cpu is not online)
> just set req->dest_cpu to -2;
> else
> init and queue req, and wake_up_process().
> preempt_enable()
> }
> for_each_possible_cpu(cpu) {
> if (req is queued)
> wait_for_completion().
> }
> mutex_unlock(&rcu_sched_expedited_mutex);

I am concerned about the following sequence of events:

o synchronize_sched_expedited() disables preemption, thus blocking
offlining operations.

o CPU 1 starts offlining CPU 0. It acquires the CPU-hotplug lock,
and proceeds, and is now waiting for preemption to be enabled.

o synchronize_sched_expedited() disables preemption, sees
that CPU 0 is online, so initializes and queues a request,
does a wake-up-process(), and finally does a preempt_enable().

o CPU 0 is currently running a high-priority real-time process,
so the wakeup does not immediately happen.

o The offlining process completes, including the kthread_stop()
to the migration task.

o The migration task wakes up, sees kthread_should_stop(),
and so exits without checking its queue.

o synchronize_sched_expedited() waits forever for CPU 0 to respond.

I suppose that one way to handle this would be to check for the CPU
going offline before doing the wait_for_completion(), but I am concerned
about races affecting this check as well.

Or is there something in the CPU-offline process that makes the above
sequence of events impossible?

Thanx, Paul