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

[Farhan Ali <alifm@xxxxxxxxxxxxx>]

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@xxxxxxxxxxxxx>
Signed-off-by: Farhan Ali <alifm@xxxxxxxxxxxxx>
---
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);
+			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:
-- 
2.7.4