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

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Date:   Wed, 27 Jan 2021 20:08:19 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Jason Wang <jasowang@...hat.com>,
        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,
        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 1/27/21 8:04 PM, Jason Wang 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).

io_uring should be fine in current kernels, but aio would still be
affected by this. But just in terms of recursion, bumping it one more
should probably still be fine.

-- 
Jens Axboe