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 11:04:42 +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 2021/1/27 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).

Thanks


>
> Thanks,
> Yongji
>