On 12/08/2016 01:06 AM, Johannes Thumshirn wrote:

On Wed, Dec 07, 2016 at 05:31:26PM -0600, Tyrel Datwyler wrote:

quoted

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: Tyrel Datwyler <redacted>
---
 drivers/scsi/ibmvscsi/ibmvscsi.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/scsi/ibmvscsi/ibmvscsi.c b/drivers/scsi/ibmvscsi/ibmvscsi.c
index d9534ee..2f5b07e 100644
--- a/drivers/scsi/ibmvscsi/ibmvscsi.c
+++ b/drivers/scsi/ibmvscsi/ibmvscsi.c

@@ -232,6 +232,7 @@ 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);

@@ -240,6 +241,7 @@ static void ibmvscsi_task(void *data)
 			vio_disable_interrupts(vdev);
 			ibmvscsi_handle_crq(crq, hostdata);
 			crq->valid = VIOSRP_CRQ_FREE;
+			wmb();
 		} else {
 			done = 1;
 		}

Is this something you have seen in the wild or just a "better save than sorry"
barrier?

I myself have not observed or heard of anybody hitting an issue here.
However, based on conversation with the VIOS developers, who have
indicated it is required, this is a "better safe than sorry" scenario.
Further, it matches what we already do in the ibmvfc driver for the CRQ
processing logic.

-Tyrel

Thanks,
	Johannes