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SubjectRe: mm: deadlock between get_online_cpus/pcpu_alloc
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On 02/07/2017 12:43 PM, Michal Hocko wrote:
>> 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();
>> }
>>
>> /*
> [...]
>> @@ -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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