Date:	Tue, 9 Dec 2014 19:25:50 +0900
From:	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
To:	Michal Hocko <mhocko@...e.cz>
CC:	"K. Y. Srinivasan" <kys@...rosoft.com>,
	<gregkh@...uxfoundation.org>, <linux-kernel@...r.kernel.org>,
	<devel@...uxdriverproject.org>, <olaf@...fle.de>,
	<apw@...onical.com>, <linux-mm@...ck.org>
Subject: Re: [PATCH 2/2] Drivers: hv: balloon: Fix the deadlock issue in the
 memory hot-add code

(2014/12/09 18:08), Michal Hocko wrote:
> On Tue 09-12-14 10:23:51, Yasuaki Ishimatsu wrote:
>> (2014/12/09 0:04), Michal Hocko wrote:
>>> On Fri 05-12-14 16:41:38, K. Y. Srinivasan wrote:
>>>> Andy Whitcroft <apw@...onical.com> initially saw this deadlock. We
>>>> have seen this as well. Here is the original description of the
>>>> problem (and a potential solution) from Andy:
>>>>
>>>> https://lkml.org/lkml/2014/3/14/451
>>>>
>>>> Here is an excerpt from that mail:
>>>>
>>>> "We are seeing machines lockup with what appears to be an ABBA
>>>> deadlock in the memory hotplug system.  These are from the 3.13.6 based Ubuntu kernels.
>>>> The hv_balloon driver is adding memory using add_memory() which takes
>>>> the hotplug lock
>>>
>>> Do you mean mem_hotplug_begin?
>>>
>>
>>>> and then emits a udev event, and then attempts to
>>>> lock the sysfs device.  In response to the udev event udev opens the
>>>> sysfs device and locks it, then attempts to grab the hotplug lock to online the memory.
>>>
>>> Cannot we simply teach online_pages to fail with EBUSY when the memory
>>> hotplug is on the way.  We shouldn't try to online something that is not
>>> initialized yet, no?
>>
>> Yes. Memory online shouldn't try before initializing it. Then memory online
>> should wait for initializing it, not easily fails. Generally, kernel sends
>> memory ONLINE event to userland by kobject_uevent() during initializing memory
>> and udev makes memory online after catching the event. Onlining memory by
>> udev almost run during initializing memory.
>
> I guess this is because the event is sent after a mem section is
> initialized while the overal hotplug operation is still not completed.

right.

>
>> So if memory online easily fails, onlining memory by udev almost
>> fails.
>
> Doesn't udev retry the operation if it gets EBUSY or EAGAIN?

It depend on implementation of udev.rules. So we can retry online/offline
operation in udev.rules.

>
>>> The memory hotplug log is global so we can get
>>> false positives but that should be easier to deal with than exporting
>>> lock_device_hotplug and adding yet another lock dependency.
>>>
>>>> This seems to be inverted nesting in the two cases, leading to the hangs below:
>>>>
>>>> [  240.608612] INFO: task kworker/0:2:861 blocked for more than 120 seconds.
>>>> [  240.608705] INFO: task systemd-udevd:1906 blocked for more than 120 seconds.
>>>>
>>>> I note that the device hotplug locking allows complete retries (via
>>>> ERESTARTSYS) and if we could detect this at the online stage it could
>>>> be used to get us out.
>>>
>>> I am not sure I understand this but it suggests EBUSY above?
>>>
>>>> But before I go down this road I wanted to
>>>> make sure I am reading this right.  Or indeed if the hv_balloon driver
>>>> is just doing this wrong."
>>>>
>>>> This patch is based on the suggestion from
>>>> Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
>>>
>>> This changelog doesn't explain us much. And boy this whole thing is so
>>> convoluted. E.g. I have hard time to see why ACPI hotplug is working
>>> correctly. My trail got lost at acpi_memory_device_add level which is
>>> a callback while acpi_device_hotplug is holding lock_device_hotplug but
>>> then again the rest is hidden by callbacks.
>>
>> Commit 0f1cfe9d0d06 (mm/hotplug: remove stop_machine() from try_offline_node()) said:
>>
>>    ---
>>      lock_device_hotplug() serializes hotplug & online/offline operations.  The
>>      lock is held in common sysfs online/offline interfaces and ACPI hotplug
>>      code paths.
>>
>>      And here are the code paths:
>>
>>      - CPU & Mem online/offline via sysfs online
>>          store_online()->lock_device_hotplug()
>>
>>      - Mem online via sysfs state:
>>          store_mem_state()->lock_device_hotplug()
>>
>>      - ACPI CPU & Mem hot-add:
>>          acpi_scan_bus_device_check()->lock_device_hotplug()
>>
>>      - ACPI CPU & Mem hot-delete:
>>          acpi_scan_hot_remove()->lock_device_hotplug()
>>    ---
>>
>> CPU & Memory online/offline/hotplug are serialized by lock_device_hotplug().
>
> OK, this patch aimed at the complete nodes hotplug. I am not familiar
> with the code enough to tell whether this is really needed but it sounds
> like an overkill when we are interested only in the memory hotplug. Why
> would we need stop_machine or anything for memory that is guaranteed to
> be not used at the time of both online and offline.

As Srinivasan mentioned, there is ABBA issue. So to evade it, lock_device_hotplug()
is needed.

> And again, why cannot we simply make the onlining fail or try_lock and
> retry internally if the event consumer cannot cope with errors?

Did you mean the following Srinivasan's first patch looks good to you?
   https://lkml.org/lkml/2014/12/2/662

Thanks,
Yasuaki Ishimatsu

> And even if that is not possible then do not export lock_device_hotplug
> but export a memory hotplug functions which use it properly so that
> other consumers (xen ballon seem to rely on add_memory as well) do not
> need the same change as well.


>
>>> I cannot seem to find any documentation which would explain all the
>>> locking here.
>>
>> Yes. I need the documentation.
>
> Yes please! This code is incredibly hard to follow and deduce all the
> hidden requirements and dependencies is even harder.
>


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