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

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Date:   Thu, 28 Jan 2021 13:12:29 +0800
From:   Yongji Xie <xieyongji@...edance.com>
To:     Jens Axboe <axboe@...nel.dk>, Jason Wang <jasowang@...hat.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: Re: [RFC v3 01/11] eventfd: track eventfd_signal() recursion
 depth separately in different cases

On Thu, Jan 28, 2021 at 11:08 AM Jens Axboe <axboe@...nel.dk> wrote:
>
> 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.
>

OK, I see. It should be easy to avoid the A-A deadlock during coding.

Thanks,
Yongji