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

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Message-ID: <6a5f0186-c2e3-4603-9826-50d5c68a3fda@redhat.com>
Date:   Wed, 27 Jan 2021 11:37:54 +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>, 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 2021/1/20 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?

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
>