Messages in this thread | | | Date | Fri, 15 Dec 2006 11:27:48 -0800 | From | Andrew Morton <> | Subject | Re: + schedule_on_each_cpu-use-preempt_disable.patch added to -mm tree |
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On Fri, 15 Dec 2006 17:24:16 +0100 Ingo Molnar <mingo@elte.hu> wrote:
> > * Andrew Morton <akpm@osdl.org> wrote: > > > > > for_each_online_cpu(cpu) { > > > > INIT_WORK(per_cpu_ptr(works, cpu), func); > > > > __queue_work(per_cpu_ptr(keventd_wq->cpu_wq, cpu), > > > > per_cpu_ptr(works, cpu)); > > > > } > > > > - mutex_unlock(&workqueue_mutex); > > > > + preempt_enable(); > > > > > > Why not cpu_hotplug_lock()? > > > > > > > Because the workqueue code was explicitly switched over to > > per-subsystem cpu-hotplug locking. > > > > Because lock_cpu_hotplug() is a complete turkey, source of deadlocks > > and overall bad idea. > > not in the locking model i outlined earlier, which would turn it into a > read-lock in essence. > > > This is actually a pretty simple problem. A subsystem has per-cpu > > reosurces, and it needs to lock them while using them. duh. We know > > how to do that sort of thing. But because the first implementation of > > lock_cpu_hotplug() was conceived with magical properties, we seem to > > think we need to retain magical properties. We don't... > > actually, we use two things here: cpu_online_map and the per-cpu keventd > workqueues. cpu_online_map is pretty much attached to the CPU hotplug > subsystem so it would be quite natural to use cpu_hotplug_read_lock() > for that.
The two are connected, because cpu add/remove creates and kills keventd threads.
> so i disagree that CPU hotplug locking should be per-subsystem. We > should have one lightweight and scalable primitive that protects > cpu_online_map use, and that same primitive can be used to protect other > per-CPU resources too.
This problem can be (is being) solved using existing locking primitives.
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