Messages in this thread | | | Date | Tue, 26 May 2009 08:46:25 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC] v7 expedited "big hammer" RCU grace periods |
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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
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