Re: [PATCH v3 1/1] vfio-ccw: Prevent quiesce function going into an infinite loop

Halil Pasic
2019-04-17T15:13:11Z

On Wed, 17 Apr 2019 09:58:24 -0400
Eric Farman <farman@linux.ibm.com> wrote:

>
>
> On 4/17/19 5:03 AM, Cornelia Huck wrote:
> > On Tue, 16 Apr 2019 17:23:14 -0400
> > Farhan Ali <alifm@linux.ibm.com> wrote:
> >
> >> The quiesce function calls cio_cancel_halt_clear() and if we
> >> get an -EBUSY we go into a loop where we:
> >> - wait for any interrupts
> >> - flush all I/O in the workqueue
> >> - retry cio_cancel_halt_clear
> >>
> >> During the period where we are waiting for interrupts or
> >> flushing all I/O, the channel subsystem could have completed
> >> a halt/clear action and turned off the corresponding activity
> >> control bits in the subchannel status word. This means the next
> >> time we call cio_cancel_halt_clear(), we will again start by
> >> calling cancel subchannel and so we can be stuck between calling
> >> cancel and halt forever.
> >>
> >> Rather than calling cio_cancel_halt_clear() immediately after
> >> waiting, let's try to disable the subchannel. If we succeed in
> >> disabling the subchannel then we know nothing else can happen
> >> with the device.
> >>
> >> Suggested-by: Eric Farman <farman@linux.ibm.com>
> >> Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
> >> ---
> >> ChangeLog:
> >> v2 -> v3
> >> - Log an error message when cio_cancel_halt_clear
> >> returns EIO and break out of the loop.
> >>
> >> - Did not include past change log as the other patches
> >> of the original series have been queued by Conny.
> >> Old series (v2) can be found here:
> >> https://marc.info/?l=kvm&m=155475754101769&w=2
> >>
> >> drivers/s390/cio/vfio_ccw_drv.c | 32 ++++++++++++++++++--------------
> >> 1 file changed, 18 insertions(+), 14 deletions(-)
> >>
> >> diff --git a/drivers/s390/cio/vfio_ccw_drv.c b/drivers/s390/cio/vfio_ccw_drv.c
> >> index 78517aa..66a66ac 100644
> >> --- a/drivers/s390/cio/vfio_ccw_drv.c
> >> +++ b/drivers/s390/cio/vfio_ccw_drv.c
> >> @@ -43,26 +43,30 @@ int vfio_ccw_sch_quiesce(struct subchannel *sch)
> >> if (ret != -EBUSY)
> >> goto out_unlock;
> >>
> >> + iretry = 255;
> >> do {
> >> - iretry = 255;
> >>
> >> ret = cio_cancel_halt_clear(sch, &iretry);
> >> - while (ret == -EBUSY) {
> >> - /*
> >> - * Flush all I/O and wait for
> >> - * cancel/halt/clear completion.
> >> - */
> >> - private->completion = &completion;
> >> - spin_unlock_irq(sch->lock);
> >>
> >> - wait_for_completion_timeout(&completion, 3*HZ);
> >> + if (ret == -EIO) {
> >> + pr_err("vfio_ccw: could not quiesce subchannel 0.%x.%04x!\n",
> >> + sch->schid.ssid, sch->schid.sch_no);
> >
> > What about using
> > dev_err(&sch->dev, "could not quiesce");
> > instead?
>
> +1
>
> >
> > (Can make that change while applying, no need to resend for that.)
> >
> >> + break;
> >> + }
> >> +
> >> + /*
> >> + * Flush all I/O and wait for
> >> + * cancel/halt/clear completion.
> >> + */
> >> + private->completion = &completion;
> >> + spin_unlock_irq(sch->lock);
> >>
> >> - private->completion = NULL;
> >> - flush_workqueue(vfio_ccw_work_q);
> >> - spin_lock_irq(sch->lock);
> >> - ret = cio_cancel_halt_clear(sch, &iretry);
> >> - };
> >> + if (ret == -EBUSY)
> >> + wait_for_completion_timeout(&completion, 3*HZ);
> >>
> >> + private->completion = NULL;
> >> + flush_workqueue(vfio_ccw_work_q);
> >> + spin_lock_irq(sch->lock);
> >> ret = cio_disable_subchannel(sch);
> >> } while (ret == -EBUSY);
> >> out_unlock:
> >
> > Otherwise, looks good to me. Will queue when I get some ack/r-b.
> >
>
> I like it, but I feel weird giving an r-b to something I suggested:
>
> Acked-by: Eric Farman <farman@linux.ibm.com>
>

I think r-b is fine. You did verify both the design and the
implementation I guess. So I don't see why not.

How urgent is this. I could give this some love till the end of the
week. Should I @Connie,@Farhan?

I was mostly ignoring these patches so I can't capitalize on my
understanding from reviewing the previous versions and need some time to
say something about it.

Regards,
Halil