linux-kernel - Re: [PATCH] virtio_mem: break device on remove

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Date:   Mon, 17 Jan 2022 09:31:56 +0100
From:   David Hildenbrand <david@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] virtio_mem: break device on remove

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@...hat.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.
-- 
Thanks,

David / dhildenb