Re: [RFC v3 01/11] eventfd: track eventfd_signal() recursion depth separately in different cases

On 2021/1/20 下午2:52, Yongji Xie wrote:

On Wed, Jan 20, 2021 at 12:24 PM Jason Wang [off-list ref] wrote:

quoted

On 2021/1/19 下午12:59, Xie Yongji wrote:

quoted

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