Re: [Xen-devel] [PATCH v1 0/6] mm: online/offline_pages called w.o. mem_hotplug_lock

[David Hildenbrand <david@xxxxxxxxxx>]

Am 19.09.18 um 03:22 schrieb Balbir Singh:
> On Tue, Sep 18, 2018 at 01:48:16PM +0200, David Hildenbrand wrote:
>> Reading through the code and studying how mem_hotplug_lock is to be used,
>> I noticed that there are two places where we can end up calling
>> device_online()/device_offline() - online_pages()/offline_pages() without
>> the mem_hotplug_lock. And there are other places where we call
>> device_online()/device_offline() without the device_hotplug_lock.
>>
>> While e.g.
>>      echo "online" > /sys/devices/system/memory/memory9/state
>> is fine, e.g.
>>      echo 1 > /sys/devices/system/memory/memory9/online
>> Will not take the mem_hotplug_lock. However the device_lock() and
>> device_hotplug_lock.
>>
>> E.g. via memory_probe_store(), we can end up calling
>> add_memory()->online_pages() without the device_hotplug_lock. So we can
>> have concurrent callers in online_pages(). We e.g. touch in online_pages()
>> basically unprotected zone->present_pages then.
>>
>> Looks like there is a longer history to that (see Patch #2 for details),
>> and fixing it to work the way it was intended is not really possible. We
>> would e.g. have to take the mem_hotplug_lock in device/base/core.c, which
>> sounds wrong.
>>
>> Summary: We had a lock inversion on mem_hotplug_lock and device_lock().
>> More details can be found in patch 3 and patch 6.
>>
>> I propose the general rules (documentation added in patch 6):
>>
>> 1. add_memory/add_memory_resource() must only be called with
>>    device_hotplug_lock.
>> 2. remove_memory() must only be called with device_hotplug_lock. This is
>>    already documented and holds for all callers.
>> 3. device_online()/device_offline() must only be called with
>>    device_hotplug_lock. This is already documented and true for now in core
>>    code. Other callers (related to memory hotplug) have to be fixed up.
>> 4. mem_hotplug_lock is taken inside of add_memory/remove_memory/
>>    online_pages/offline_pages.
>>
>> To me, this looks way cleaner than what we have right now (and easier to
>> verify). And looking at the documentation of remove_memory, using
>> lock_device_hotplug also for add_memory() feels natural.
>>
> 
> That seems reasonable, but also implies that device_online() would hold
> back add/remove memory, could you please also document what mode
> read/write the locks need to be held? For example can the device_hotplug_lock
> be held in read mode while add/remove memory via (mem_hotplug_lock) is held
> in write mode?

device_hotplug_lock is an ordinary mutex. So no option there.

Only mem_hotplug_lock is a per CPU RW mutex. And as of now it only
exists to not require get_online_mems()/put_online_mems() to take the
device_hotplug_lock. Which is perfectly valid, because these users only
care about memory (not any other devices) not suddenly vanish. And that
RW lock makes things fast.

Any modifications (online/offline/add/remove) require the
mem_hotplug_lock in write.

I can add some more details to documentation in patch #6.

"... we should always hold the mem_hotplug_lock (via
mem_hotplug_begin/mem_hotplug_done) in write mode to serialize memory
hotplug" ..."

"In addition, mem_hotplug_lock (in contrast to device_hotplug_lock) in
read mode allows for a quite efficient get_online_mems/put_online_mems
implementation, so code accessing memory can protect from that memory
vanishing."

Would that work for you?

Thanks!

> 
> Balbir Singh.
>  
> 


-- 

Thanks,

David / dhildenb

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