Virtualization - May 2022 - [PATCH V4 6/9] virtio-ccw: implement synchronize_cbs()

On Wed, May 11 2022, Jason Wang <jasowang at redhat.com> wrote:
> On Tue, May 10, 2022 at 7:28 PM Michael S. Tsirkin <mst at
redhat.com> wrote:
>>
>> On Sat, May 07, 2022 at 03:19:51PM +0800, Jason Wang wrote:
>> > @@ -1106,6 +1130,7 @@ static void virtio_ccw_int_handler(struct
ccw_device *cdev,
>> >                       vcdev->err = -EIO;
>> >       }
>> >       virtio_ccw_check_activity(vcdev, activity);
>> > +     read_lock_irqsave(&vcdev->irq_lock, flags);
>> >       for_each_set_bit(i, indicators(vcdev),
>> >                        sizeof(*indicators(vcdev)) * BITS_PER_BYTE)
{
>> >               /* The bit clear must happen before the vring kick.
*/
>>
>> Cornelia sent a lockdep trace on this.
>>
>> Basically I think this gets the irqsave/restore logic wrong.
>> It attempts to disable irqs in the handler (which is an interrupt
>> anyway).
>
> The reason I use irqsave/restore is that it can be called from process
> context (if I was not wrong), e.g from io_subchannel_quiesce().
io_subchannel_quiesce() should disable interrupts, though? Otherwise, it
would be a bug.

On Wed, May 11, 2022 at 4:17 PM Cornelia Huck <cohuck at redhat.com>
wrote:
>
> On Wed, May 11 2022, Jason Wang <jasowang at redhat.com> wrote:
>
> > On Tue, May 10, 2022 at 7:28 PM Michael S. Tsirkin <mst at
redhat.com> wrote:
> >>
> >> On Sat, May 07, 2022 at 03:19:51PM +0800, Jason Wang wrote:
> >> > @@ -1106,6 +1130,7 @@ static void
virtio_ccw_int_handler(struct ccw_device *cdev,
> >> >                       vcdev->err = -EIO;
> >> >       }
> >> >       virtio_ccw_check_activity(vcdev, activity);
> >> > +     read_lock_irqsave(&vcdev->irq_lock, flags);
> >> >       for_each_set_bit(i, indicators(vcdev),
> >> >                        sizeof(*indicators(vcdev)) *
BITS_PER_BYTE) {
> >> >               /* The bit clear must happen before the vring
kick. */
> >>
> >> Cornelia sent a lockdep trace on this.
> >>
> >> Basically I think this gets the irqsave/restore logic wrong.
> >> It attempts to disable irqs in the handler (which is an interrupt
> >> anyway).
> >
> > The reason I use irqsave/restore is that it can be called from process
> > context (if I was not wrong), e.g from io_subchannel_quiesce().
>
> io_subchannel_quiesce() should disable interrupts, though? Otherwise, it
> would be a bug.
Right, it was protected by a spin_lock_irq(), but I can see other
cdev->handler() in e.g device_fsm.c, the irq status is not obvious, do
they have the same assumption which IRQ is disabled?

Thanks
>