Re: [PATCH 2/2] virtio-blk: Ensure no requests in virtqueues before deleting vqs.

[Stefan Hajnoczi]

Hi Michael,
This could potentially affect other VIRTIO drivers too. Please see
below.

On Tue, Jan 23, 2024 at 11:27:40AM +0800, yi sun wrote:
> On Mon, Jan 22, 2024 at 11:43 PM Stefan Hajnoczi <stefanha@xxxxxxxxxx> wrote:
> >
> > On Mon, Jan 22, 2024 at 07:07:22PM +0800, Yi Sun wrote:
> > > Ensure no remaining requests in virtqueues before resetting vdev and
> > > deleting virtqueues. Otherwise these requests will never be completed.
> > > It may cause the system to become unresponsive. So it is better to place
> > > blk_mq_quiesce_queue() in front of virtio_reset_device().
> >
> > QEMU's virtio-blk device implementation completes all requests during
> > device reset. Most device implementations have to do the same to avoid
> > leaving dangling requests in flight across device reset.
> >
> > Which device implementation are you using and why is it safe for the
> > device is simply drop requests across device reset?
> >
> > Stefan
> 
> Virtio-blk device implementation completes all requests during device reset, but
> this can only ensure that the device has finished using virtqueue. We should
> also consider the driver's use of virtqueue.
> 
> I caught such an example. Before del_vqs, the request had been processed by
> the device, but it had not been processed by the driver. Although I am
> using kernel5.4,
> I think this problem may also occur with the latest version of kernel.
> 
> The debug code I added is as follows:
> virtblk_freeze()
> {
>         vdev reset();
>         quiesce queue();
>         if (virtqueue->num_free != 1024) //1024 is the init value.
>                 BUG_ON(1);
>         vdev del_vqs();
> }
> 
> BUG_ON triggered the dump, the analysis is as follows:
> 
> There is one request left in request_queue.
> crash_arm64> struct request ffffff81f0560000 | grep -e state -e __data_len
>   __data_len = 20480,
>   state = MQ_RQ_IN_FLIGHT,
> 
> crash_arm64> vring_virtqueue.packed,last_used_idx,broken,vq 0xffffff8086f92900 |
> grep -e num -e used_wrap_counter -e last_used_idx -e broken -e
> num_free -e desc_state -e "desc ="
>         num = 1024,
>         desc = 0xffffff8085ff8000,
>       used_wrap_counter = false,
>       desc_state = 0xffffff8085610000,
>   last_used_idx = 487,
>   broken = false,
>     num_free = 1017,
> 
> Find desc based on last_used_idx. Through flags, we can know that the request
> has been processed by the device, but it is still in flight state
> because it has not
> had time to run virtblk_done().
> crash_arm> vring_packed_desc ffffff8085ff9e70
> struct vring_packed_desc {
>   addr = 10474619192,
>   len = 20481,
>   id = 667,
>   flags = 2
> }
> 
> I'm using a closed source virtual machine, so I can't see the source
> code for it,
> but I'm guessing it's similar to qemu.
> 
> After the device completes the request, we must also ensure that the driver can
> complete the request in virtblk_done().
> 

Okay, I think your approach of waiting for requests before
virtio_device_reset() makes sense. blk_mq_complete_request() is async
(might be deferred to an IPI or softirq) so it's not enough for
virtblk_done() to run before virtio_device_reset() returns. There is no
guarantee that virtblk_request_done() runs before virtio_device_reset()
returns.

A separate issue about virtio_device_reset():

Are you using virtio-mmio? virtio-mmio's vm_reset() doesn't offer the
same guarantees as virtio-pci's reset functions. virtio-pci guarantees
that irqs sent by the device during the reset operation complete and the
irq handlers run before virtio_device_reset() returns. virtio-mmio does
not.

(On top of this, virtio_device_reset() now has
CONFIG_VIRTIO_HARDEN_NOTIFICATION which changes the semantics. Drivers
cannot expect to complete any in-flight requests in
virtio_device_reset() when complied with this config option.)

Other drivers may be affected by these inconsistent
virtio_device_reset() semantics. I haven't audited the code.

Stefan

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