On 02/07/2017 12:43 PM, Michal Hocko wrote:

quoted

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3b93879990fd..7af165d308c4 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c

@@ -2342,7 +2342,14 @@ void drain_local_pages(struct zone *zone)

 static void drain_local_pages_wq(struct work_struct *work)
 {
+	/*
+	 * Ordinarily a drain operation is bound to a CPU but may be unbound
+	 * after a CPU hotplug operation so it's necessary to disable
+	 * preemption for the drain to stabilise the CPU ID.
+	 */
+	preempt_disable();
 	drain_local_pages(NULL);
+	preempt_enable_no_resched();
 }

 /*

[...]

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?

Could we do flush_work() with a spinlock? Sounds bad too.
Maybe we could just use the fact that the whole drain happens with disabled 
irq's and obtain the current cpu under that protection?

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