Paul E. McKenney wrote: > > 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 > > I realized this, I wrote this: > > 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 synchronize_sched_expedited() will not wait for CPU#0, because migration_call()::case CPU_DEAD wakes up the requestors. migration_call() { ... case CPU_DEAD: case CPU_DEAD_FROZEN: ... /* * No need to migrate the tasks: it was best-effort if * they didn't take sched_hotcpu_mutex. Just wake up * the requestors. */ spin_lock_irq(&rq->lock); while (!list_empty(&rq->migration_queue)) { struct migration_req *req; req = list_entry(rq->migration_queue.next, struct migration_req, list); list_del_init(&req->list); spin_unlock_irq(&rq->lock); complete(&req->done); spin_lock_irq(&rq->lock); } spin_unlock_irq(&rq->lock); ... ... } My approach depend on the requestors are waked up at any case. migration_call() does it for us but the coupling is largely increased. Lai -- To unsubscribe from this list: send the line "unsubscribe netfilter-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html