On 02/07/2017 01:37 PM, Michal Hocko wrote:

quoted

quoted

@@ -6711,7 +6714,16 @@ static int page_alloc_cpu_dead(unsigned int cpu)
 {

 	lru_add_drain_cpu(cpu);
+
+	/*
+	 * A per-cpu drain via a workqueue from drain_all_pages can be
+	 * rescheduled onto an unrelated CPU. That allows the hotplug
+	 * operation and the drain to potentially race on the same
+	 * CPU. Serialise hotplug versus drain using pcpu_drain_mutex
+	 */
+	mutex_lock(&pcpu_drain_mutex);
 	drain_pages(cpu);
+	mutex_unlock(&pcpu_drain_mutex);

You cannot put sleepable lock inside the preempt disbaled section...
We can make it a spinlock right?

Scratch that! For some reason I thought that cpu notifiers are run in an
atomic context. Now that I am checking the code again it turns out I was
wrong. __cpu_notify uses __raw_notifier_call_chain so this is not an
atomic context.

Good.

Anyway, shouldn't be it sufficient to disable preemption
on drain_local_pages_wq? The CPU hotplug callback will not preempt us
and so we cannot work on the same cpus, right?

I thought the problem here was that the callback races with the work item that 
has been migrated to a different cpu. Once we are not working on the local cpu, 
disabling preempt/irq's won't help?

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