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Subject: Re: WARNING in __mmdrop
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From:   Jason Wang <jasowang@redhat.com>
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Date:   Tue, 23 Jul 2019 15:53:06 +0800
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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?


>
>>>> Btw, for kvm ioctl it still uses synchronize_rcu() in kvm_vcpu_ioctl(),
>>>> (just a little bit more hard to trigger):
>>> AFAIK these never run in response to guest events.
>>> So they can take very long and guests still won't crash.
>> What if guest manages to escape to qemu?
>>
>> Thanks
> Then it's going to be slow. Why do we care?
> What we do not want is synchronize_rcu that guest is blocked on.
>

Ok, this looks like that I have some misunderstanding here of the reason 
why synchronize_rcu() is not preferable in the path of ioctl. But in kvm 
case, if rcu_expedited is set, it can triggers IPIs AFAIK.

Thanks