vfio-ccw: Prevent quiesce function going into an infinite loop

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>
Message-Id: <4d5a4b98ab1b41ac6131b5c36de18b76c5d66898.1555449329.git.alifm@linux.ibm.com>
Reviewed-by: Eric Farman <farman@linux.ibm.com>
Acked-by: Halil Pasic <pasic@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>

diff --git a/drivers/s390/cio/vfio_ccw_drv.c b/drivers/s390/cio/vfio_ccw_drv.c
index d72aa8c760c5711a979ef4e6d5d4a7653ad53656..ee8767f5845ae8c6050235b2f08ede977c539332 100644 (file)
--- 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: