On Thu, Jan 28, 2021 at 11:05 AM Jason Wang <jasowang@xxxxxxxxxx> wrote:
>
>
> On 2021/1/27 下午5:11, Yongji Xie wrote:
> > On Wed, Jan 27, 2021 at 11:38 AM Jason Wang <jasowang@xxxxxxxxxx> wrote:
> >>
> >> On 2021/1/20 下午2:52, Yongji Xie wrote:
> >>> On Wed, Jan 20, 2021 at 12:24 PM Jason Wang <jasowang@xxxxxxxxxx> wrote:
> >>>> On 2021/1/19 下午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?
> >>
> > Actually I mean the deadlock described in commit f0b493e ("io_uring:
> > prevent potential eventfd recursion on poll"). It can break this bug
> > fix if we just increase EVENTFD_WAKEUP_DEPTH.
>
>
> Ok, so can wait do something similar in that commit? (using async stuffs
> like wq).
>
We can do that. But it will reduce the performance. Because the
eventfd recursion will be triggered every time kvm kick eventfd in
vhost-vdpa cases:
KVM write KICKFD -> ops->kick_vq -> VDUSE write KICKFD
Thanks,
Yongji
