On Mon, Aug 02, 2021 at 05:01:14PM +0200, Laurent Vivier wrote:
> On 30/07/2021 03:58, AIERPATIJIANG1 [艾尔帕提江·阿布都赛买提] wrote:
> > Ports enter a "throttled" state when writing to the chardev would block.
> > The current output VirtQueueElement is kept around until the chardev
> > becomes writable again.
> >
> > Because closing the virtio serial device does not reset the queue, we cannot
> > directly discard this element, otherwise the control variables of the front
> > and back ends of the queue are inconsistent such as used_index. We should
> > unpop the
> > VirtQueueElement to queue, let discard_vq_data process it.
> >
> > The test environment:
> > kernel: linux-5.12
> > Qemu command:
> > Qemu-system-x86 -machine pc,accel=kvm \
> > -cpu host,host-phys-bits \
> > -smp 4 \
> > -m 4G \
> > -kernel ./kernel \
> > -display none \
> > -nodefaults \
> > -serial mon:stdio \
> > -append "panic=1 no_timer_check noreplace-smp rootflags=data=ordered
> > rootfstype=ext4 console=ttyS0 reboot=k root=/dev/vda1 rw" \
> > -drive id=os,file=./disk,if=none \
> > -device virtio-blk-pci,drive=os \
> > -device virtio-serial-pci,id=virtio-serial0,bus=pci.0,addr=0x4 \
> > -chardev socket,id=charchannel0,path=/tmp/char-dev-test,server,nowait \
> > -device
> > virtserialport,bus=virtio-serial0.0,nr=1,chardev=charchannel0,id=channel0,name=org.qemu.guest_agent.0
> >
> > full up virtio queue after VM started:
> > Cat /large-file > /dev/vport1p1
> >
> > Host side:
> > Open and close character device sockets repeatedly
> >
> > After awhile we can’t write any request to /dev/vport1p1 at VM side, VM
> > kernel soft lockup at drivers/char/virtio_console.c: __send_to_port
> >
> >
> > Signed-off-by: Arafatms <aierpatijia...@kingsoft.com>
> >
> > diff --git a/hw/char/virtio-serial-bus.c b/hw/char/virtio-serial-bus.c
> > index dd6bc27b3b..36236defdf 100644
> > --- a/hw/char/virtio-serial-bus.c
> > +++ b/hw/char/virtio-serial-bus.c
> > @@ -150,8 +150,12 @@ static void discard_vq_data(VirtQueue *vq,
> > VirtIODevice *vdev)
> >
> > static void discard_throttle_data(VirtIOSerialPort *port)
> > {
> > + if (!virtio_queue_ready(port->ovq)) {
> > + return;
> > + }
>
> Why do we need to check virtio_queue_ready() now?
I'm not sure why this is necessary either. We don't need the vring to be
functional, we're just trying to clean up our internal state.
> > +
> > if (port->elem) {
> > - virtqueue_detach_element(port->ovq, port->elem, 0);
> > + virtqueue_unpop(port->ovq, port->elem, 0);
> > g_free(port->elem);
> > port->elem = NULL;
> > }
> >
>
> It seems the problem you report can only happen when the port is closed from
> the host side
> (because from the guest side I guess the cleanup is done by the guest driver).
>
> Stefan, you have introduced the function discard_throttle_data() in:
>
> d4c19cdeeb2f ("virtio-serial: add missing virtio_detach_element() call")
>
> do you remember why you prefered to use virtqueue_detach_element() rather than
> virtqueue_unpop() (or virtqueue_discard() at this time, see 27e57efe32f5
> ("virtio: rename
> virtqueue_discard to virtqueue_unpop"))
I don't remember. virtqueue_unpop() cannot handle out-of-order element
processing, but it seems safe to use here since port->elem is the last
element we pop off ovq.
It's probably just a bug in my commit d4c19cdeeb2f. I may have thought
the virtqueue is never touched again after reset (true), close (false),
and remove port (false?).
Please add:
Fixes: d4c19cdeeb2f ("virtio-serial: add missing virtio_detach_element()
call")
Stefan
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