[RFC v2 2/3] vfio-ccw: Prevent quiesce function going into an infinite loop

Farhan Ali
2019-04-08T21:05:32Z

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>
---
drivers/s390/cio/vfio_ccw_drv.c | 27 ++++++++++++---------------
1 file changed, 12 insertions(+), 15 deletions(-)

diff --git a/drivers/s390/cio/vfio_ccw_drv.c b/drivers/s390/cio/vfio_ccw_drv.c
index 5aca475..4405f2a 100644
--- a/drivers/s390/cio/vfio_ccw_drv.c
+++ b/drivers/s390/cio/vfio_ccw_drv.c
@@ -43,26 +43,23 @@ 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);
-
+ /*
+ * Flush all I/O and wait for
+ * cancel/halt/clear completion.
+ */
+ private->completion = &completion;
+ spin_unlock_irq(sch->lock);
+
+ 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_cancel_halt_clear(sch, &iretry);
- };
-
+ private->completion = NULL;
+ flush_workqueue(vfio_ccw_work_q);
+ spin_lock_irq(sch->lock);
ret = cio_disable_subchannel(sch);
} while (ret == -EBUSY);
out_unlock:
--
2.7.4