Messages in this thread | | | Date | Mon, 1 Mar 2010 16:37:50 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH 10/43] stop_machine: reimplement without using workqueue |
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Hello,
On 03/02, Tejun Heo wrote: > > > and more importantly, if it was possible > > stop_machine_cpu_callback(CPU_POST_DEAD) (which is called after > > cpu_hotplug_done()) could race with stop_machine(). > > stop_machine_cpu_callback(CPU_POST_DEAD) relies on fact that this > > thread has already called schedule() and it can't be woken until > > kthread_stop() sets ->should_stop. > > Hmmm... I'm probably missing something but I don't see how > stop_machine_cpu_callback(CPU_POST_DEAD) depends on stop_cpu() thread > already parked in schedule(). Can you elaborate a bit?
Suppose that, when stop_machine_cpu_callback(CPU_POST_DEAD) is called, that stop_cpu() thread T is still running and it is going to check state before schedule().
CPU_POST_DEAD is called after cpu_hotplug_done(), another CPU can do stop_machine() and set STOPMACHINE_PREPARE.
If T sees state == STOPMACHINE_PREPARE it will join the game, but it wasn't counted in thread_ack counter, it is not cpu-bound, etc.
> >> int __stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus) > >> { > >> ... > >> /* Schedule the stop_cpu work on all cpus: hold this CPU so one > >> * doesn't hit this CPU until we're ready. */ > >> get_cpu(); > >> + for_each_online_cpu(i) > >> + wake_up_process(*per_cpu_ptr(stop_machine_threads, i)); > > > > I think the comment is wrong, and we need preempt_disable() instead > > of get_cpu(). We shouldn't worry about this CPU, but we need to ensure > > the woken real-time thread can't preempt us until we wake up them all. > > get_cpu() and preempt_disable() are exactly the same thing, aren't > they?
Yes,
> Do you think get_cpu() is wrong there for some reason?
No. I think that the comment is confusing, and preempt_disable() "looks" more correct.
In any case, this is very minor, please ignore. In fact, I mentioned this only because this email was much longer initially, at first I thought I noticed the bug, but I was wrong ;)
Oleg.
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