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Subject: Re: WARNING in __mmdrop
To:     "Michael S. Tsirkin" <mst@redhat.com>
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <caff362a-e208-3468-3688-63e1d093a9d3@redhat.com>
Date:   Tue, 23 Jul 2019 21:31:35 +0800
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On 2019/7/23 下午5:26, Michael S. Tsirkin wrote:
> On Tue, Jul 23, 2019 at 04:49:01PM +0800, Jason Wang wrote:
>> On 2019/7/23 下午4:10, Michael S. Tsirkin wrote:
>>> On Tue, Jul 23, 2019 at 03:53:06PM +0800, Jason Wang wrote:
>>>> On 2019/7/23 下午3:23, Michael S. Tsirkin wrote:
>>>>>>> Really let's just use kfree_rcu. It's way cleaner: fire and forget.
>>>>>> Looks not, you need rate limit the fire as you've figured out?
>>>>> See the discussion that followed. Basically no, it's good enough
>>>>> already and is only going to be better.
>>>>>
>>>>>> And in fact,
>>>>>> the synchronization is not even needed, does it help if I leave a comment to
>>>>>> explain?
>>>>> Let's try to figure it out in the mail first. I'm pretty sure the
>>>>> current logic is wrong.
>>>> Here is what the code what to achieve:
>>>>
>>>> - The map was protected by RCU
>>>>
>>>> - Writers are: MMU notifier invalidation callbacks, file operations (ioctls
>>>> etc), meta_prefetch (datapath)
>>>>
>>>> - Readers are: memory accessor
>>>>
>>>> Writer are synchronized through mmu_lock. RCU is used to synchronized
>>>> between writers and readers.
>>>>
>>>> The synchronize_rcu() in vhost_reset_vq_maps() was used to synchronized it
>>>> with readers (memory accessors) in the path of file operations. But in this
>>>> case, vq->mutex was already held, this means it has been serialized with
>>>> memory accessor. That's why I think it could be removed safely.
>>>>
>>>> Anything I miss here?
>>>>
>>> So invalidate callbacks need to reset the map, and they do
>>> not have vq mutex. How can they do this and free
>>> the map safely? They need synchronize_rcu or kfree_rcu right?
>> Invalidation callbacks need but file operations (e.g ioctl) not.
>>
>>
>>> And I worry somewhat that synchronize_rcu in an MMU notifier
>>> is a problem, MMU notifiers are supposed to be quick:
>> Looks not, since it can allow to be blocked and lots of driver depends on
>> this. (E.g mmu_notifier_range_blockable()).
> Right, they can block. So why don't we take a VQ mutex and be
> done with it then? No RCU tricks.


This is how I want to go with RFC and V1. But I end up with deadlock 
between vq locks and some MM internal locks. So I decide to use RCU 
which is 100% under the control of vhost.

Thanks