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Date:   Mon, 17 Jan 2022 03:40:42 -0500
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Jason Wang <jasowang@redhat.com>, linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org
Subject: Re: [PATCH] virtio_mem: break device on remove
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On Mon, Jan 17, 2022 at 09:31:56AM +0100, David Hildenbrand wrote:
> On 17.01.22 08:55, Michael S. Tsirkin wrote:
> > On Mon, Jan 17, 2022 at 02:40:11PM +0800, Jason Wang wrote:
> >>
> >> 在 2022/1/15 上午5:43, Michael S. Tsirkin 写道:
> >>> A common pattern for device reset is currently:
> >>> vdev->config->reset(vdev);
> >>> .. cleanup ..
> >>>
> >>> reset prevents new interrupts from arriving and waits for interrupt
> >>> handlers to finish.
> >>>
> >>> However if - as is common - the handler queues a work request which is
> >>> flushed during the cleanup stage, we have code adding buffers / trying
> >>> to get buffers while device is reset. Not good.
> >>>
> >>> This was reproduced by running
> >>> 	modprobe virtio_console
> >>> 	modprobe -r virtio_console
> >>> in a loop, and this reasoning seems to apply to virtio mem though
> >>> I could not reproduce it there.
> >>>
> >>> Fix this up by calling virtio_break_device + flush before reset.
> >>>
> >>> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> >>> ---
> >>>   drivers/virtio/virtio_mem.c | 2 ++
> >>>   1 file changed, 2 insertions(+)
> >>>
> >>> diff --git a/drivers/virtio/virtio_mem.c b/drivers/virtio/virtio_mem.c
> >>> index 38becd8d578c..33b8a118a3ae 100644
> >>> --- a/drivers/virtio/virtio_mem.c
> >>> +++ b/drivers/virtio/virtio_mem.c
> >>> @@ -2888,6 +2888,8 @@ static void virtio_mem_remove(struct virtio_device *vdev)
> >>>   		virtio_mem_deinit_hotplug(vm);
> >>>   	/* reset the device and cleanup the queues */
> >>> +	virtio_break_device(vdev);
> >>> +	flush_work(&vm->wq);
> >>
> >>
> >> We set vm->removing to true and call cancel_work_sync() in
> >> virtio_mem_deinit_hotplug(). Isn't is sufficient?
> >>
> >> Thanks
> > 
> > 
> > Hmm I think you are right. David, I will drop this for now.
> > Up to you to consider whether some central capability will be
> > helpful as a replacement for the virtio-mem specific "removing" flag.
> 
> It's all a bit tricky because we also have to handle pending timers and
> pending memory onlining/offlining operations in a controlled way. Maybe
> we could convert to virtio_break_device() and use the
> &dev->vqs_list_lock as a replacement for the removal_lock. However, I'm
> not 100% sure if it's nice to use that lock from
> drivers/virtio/virtio_mem.c directly.

We could add an API if you like. Or maybe it makes sense to add a
separate one that lets you find out that removal started. Need to figure
out how to handle suspend too then ...
Generally we have these checks that device is not going away
sprinkled over all drivers and I don't like it, but
it's not easy to build a sane API to handle it, especially
for high speed things when we can't take locks because performance.

> -- 
> Thanks,
> 
> David / dhildenb