Re: [RFC v4 2/3] memory: add depth assert in address_space_to_flatview

[Chuang Xu]

Hi, Peter and Paolo,
I'm sorry I didn't reply to your email in time. I was infected with
COVID-19 two weeks ago, so I couldn't think about the problems discussed
in your email for a long time.😷

On 2023/1/4 上午1:43, Peter Xu wrote:
> Hi, Paolo,
>
> On Wed, Dec 28, 2022 at 09:27:50AM +0100, Paolo Bonzini wrote:
>> Il ven 23 dic 2022, 16:54 Peter Xu ha scritto:
>>
>>>> This is not valid because the transaction could happen in *another*
>>> thread.
>>>> In that case memory_region_transaction_depth() will be > 0, but RCU is
>>>> needed.
>>> Do you mean the code is wrong, or the comment? Note that the code has
>>> checked rcu_read_locked() where introduced in patch 1, but maybe something
>>> else was missed?
>>>
>> The assertion is wrong. It will succeed even if RCU is unlocked in this
>> thread but a transaction is in progress in another thread.
> IIUC this is the case where the context:
>
> (1) doesn't have RCU read lock held, and,
> (2) doesn't have BQL held.
>
> Is it safe at all to reference any flatview in such a context? The thing
> is I think the flatview pointer can be freed anytime if both locks are not
> taken.
>
>> Perhaps you can check (memory_region_transaction_depth() > 0 &&
>> !qemu_mutex_iothread_locked()) || rcu_read_locked() instead?
> What if one thread calls address_space_to_flatview() with BQL held but not
> RCU read lock held? I assume it's a legal operation, but it seems to be
> able to trigger the assert already?
>
> Thanks,
>
I'm not sure whether I understand the content of your discussion correctly,
so here I want to explain my understanding firstly.

From my perspective, Paolo thinks that when thread 1 is in a transaction,
thread 2 will trigger the assertion when accessing the flatview without
holding RCU read lock, although sometimes the thread 2's access to flatview
is legal. So Paolo suggests checking (memory_region_transaction_depth() > 0
&& !qemu_mutex_iothread_locked()) || rcu_read_locked() instead.

And Peter thinks that as long as no thread holds the BQL or RCU read lock,
the old flatview can be released (actually executed by the rcu thread with
BQL held). When thread 1 is in a transaction, if thread 2 access the flatview
with BQL held but not RCU read lock held, it's a legal operation. In this
legal case, it seems that both my code and Paolo's code will trigger assertion.

I'm not sure if I have a good understanding of your emails? I think
checking(memory_region_transaction_get_depth() == 0 || rcu_read_locked() ||
qemu_mutex_iothread_locked()) should cover the case you discussed.

What's your take?

Thanks!