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

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Message-ID: <CACycT3sqDgccOfNcY_FNcHDqJ2DeMbigdFuHYm9DxWWMjkL7CQ@mail.gmail.com>
Date:   Wed, 27 Jan 2021 17:11:19 +0800
From:   Yongji Xie <xieyongji@...edance.com>
To:     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,
        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: Re: [RFC v3 01/11] eventfd: track eventfd_signal() recursion
 depth separately in different cases

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.

Thanks,
Yongji