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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Fri, 6 Sep 2019 15:55:21 +0200
Message-ID: <CAJfpegsL4PLvROr58vtjmyvQu-F17X3xoKCztP2H0fog0xUXhA@mail.gmail.com>
Subject: Re: [PATCH 00/18] virtiofs: Fix various races and cleanups round 1
To:     Vivek Goyal <vgoyal@redhat.com>
Cc:     Stefan Hajnoczi <stefanha@redhat.com>,
        linux-fsdevel@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, virtio-fs@redhat.com,
        "Dr. David Alan Gilbert" <dgilbert@redhat.com>,
        "Michael S. Tsirkin" <mst@redhat.com>
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On Fri, Sep 6, 2019 at 2:08 PM Vivek Goyal <vgoyal@redhat.com> wrote:
>
> On Fri, Sep 06, 2019 at 01:52:41PM +0200, Miklos Szeredi wrote:
> > On Fri, Sep 6, 2019 at 12:36 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > >
> > > On Fri, Sep 06, 2019 at 10:15:14AM +0200, Miklos Szeredi wrote:
> > > > On Thu, Sep 5, 2019 at 9:49 PM Vivek Goyal <vgoyal@redhat.com> wrote:
> > > > >
> > > > > Hi,
> > > > >
> > > > > Michael Tsirkin pointed out issues w.r.t various locking related TODO
> > > > > items and races w.r.t device removal.
> > > > >
> > > > > In this first round of cleanups, I have taken care of most pressing
> > > > > issues.
> > > > >
> > > > > These patches apply on top of following.
> > > > >
> > > > > git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse.git#virtiofs-v4
> > > > >
> > > > > I have tested these patches with mount/umount and device removal using
> > > > > qemu monitor. For example.
> > > >
> > > > Is device removal mandatory?  Can't this be made a non-removable
> > > > device?  Is there a good reason why removing the virtio-fs device
> > > > makes sense?
> > >
> > > Hot plugging and unplugging virtio PCI adapters is common.  I'd very
> > > much like removal to work from the beginning.
> >
> > Can you give an example use case?
>
> David Gilbert mentioned this could be useful if daemon stops responding
> or dies. One could remove device. That will fail all future requests
> and allow unmounting filesystem.
>
> Havind said that, current implementation will help in above situation
> only if there are no pending requests. If there are pending requests
> and daemon stops responding, then removal will hang too, as we wait
> for draining the queues.
>
> So at some point of time, we also need some sort of timeout functionality
> where we end requests with error after a timeout.
>
> I feel we should support removing device at some point of time. But its
> not necessarily a must have feature for first round.

If there's no compelling reason to do it in the first round, than I'd
prefer to not do it.   More complexity -> more bugs.

Thanks,
Miklos