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

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Date:   Thu, 28 Jan 2021 12:31:01 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Yongji Xie <xieyongji@...edance.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Stefano Garzarella <sgarzare@...hat.com>,
        Parav Pandit <parav@...dia.com>, Bob Liu <bob.liu@...cle.com>,
        Christoph Hellwig <hch@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Matthew Wilcox <willy@...radead.org>, viro@...iv.linux.org.uk,
        axboe@...nel.dk, bcrl@...ck.org, Jonathan Corbet <corbet@....net>,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        kvm@...r.kernel.org, linux-aio@...ck.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [RFC v3 01/11] eventfd: track eventfd_signal() recursion depth
 separately in different cases


On 2021/1/28 上午11:52, Yongji Xie wrote:
> On Thu, Jan 28, 2021 at 11:05 AM Jason Wang <jasowang@...hat.com> wrote:
>>
>> On 2021/1/27 下午5:11, Yongji Xie wrote:
>>> On Wed, Jan 27, 2021 at 11:38 AM Jason Wang <jasowang@...hat.com> wrote:
>>>> On 2021/1/20 下午2:52, Yongji Xie wrote:
>>>>> On Wed, Jan 20, 2021 at 12:24 PM Jason Wang <jasowang@...hat.com> 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


Right, I think in the future we need to find a way to let KVM to wakeup 
VDUSE directly.

Havn't had a deep thought but it might work like irq bypass manager.

Thanks