scsi: ibmvscsi: add write memory barrier to CRQ processing

The first byte of each CRQ entry is used to indicate whether an entry is
a valid response or free for the VIOS to use. After processing a
response the driver sets the valid byte to zero to indicate the entry is
now free to be reused. Add a memory barrier after this write to ensure
no other stores are reordered when updating the valid byte.
Signed-off-by: NTyrel Datwyler <tyreld@linux.vnet.ibm.com>
Reviewed-by: NBrian King <brking@linux.vnet.ibm.com>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/ibmvscsi/ibmvscsi.c

@@ -233,6 +233,7 @@ static void ibmvscsi_task(void *data)
 		while ((crq = crq_queue_next_crq(&hostdata->queue)) != NULL) {
 			ibmvscsi_handle_crq(crq, hostdata);
 			crq->valid = VIOSRP_CRQ_FREE;
+			wmb();
 		}
 
 		vio_enable_interrupts(vdev);
@@ -241,6 +242,7 @@ static void ibmvscsi_task(void *data)
 			vio_disable_interrupts(vdev);
 			ibmvscsi_handle_crq(crq, hostdata);
 			crq->valid = VIOSRP_CRQ_FREE;
+			wmb();
 		} else {
 			done = 1;
 		}