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

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Date:   Wed, 20 Jan 2021 14:52:38 +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>, 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 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. 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