Linux Virtualization - Jan 2021 - [RFC v3 01/11] eventfd: track eventfd_signal() recursion depth separately in different cases

On 2021/1/20 ??2:52, Yongji Xie wrote:
> On Wed, Jan 20, 2021 at 12:24 PM Jason Wang <jasowang at redhat.com>
wrote:
>>
>> On 2021/1/19 ??12:59, Xie Yongji wrote:
>>> Now we have a global percpu counter to limit the recursion depth
>>> of eventfd_signal(). This can avoid deadlock or stack overflow.
>>> But in stack overflow case, it should be OK to increase the
>>> recursion depth if needed. So we add a percpu counter in
eventfd_ctx
>>> to limit the recursion depth for deadlock case. Then it could be
>>> fine to increase the global percpu counter later.
>>
>> I wonder whether or not it's worth to introduce percpu for each
eventfd.
>>
>> How about simply check if eventfd_signal_count() is greater than 2?
>>
> It can't avoid deadlock in this way.

I may miss something but the count is to avoid recursive eventfd call. 
So for VDUSE what we suffers is e.g the interrupt injection path:

userspace write IRQFD -> vq->cb() -> another IRQFD.

It looks like increasing EVENTFD_WAKEUP_DEPTH should be sufficient?

Thanks

> So we need a percpu counter for
> each eventfd to limit the recursion depth for deadlock cases. And
> using a global percpu counter to avoid stack overflow.
>
> Thanks,
> Yongji
>