Subject: Re: [PATCH] ibmvscsi: add write memory barrier to CRQ processing

From: Benjamin Herrenschmidt <[email-masked]>
Date: Sat, 10 Dec 2016 08:20:52 +1100

On Wed, 2016-12-07 at 17:31 -0600, Tyrel Datwyler wrote:
> 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.

Which "other stores" specifically ? This smells fishy without that
precision. It's important to always understand what exactly barriers
order with.

Cheers,
Ben.

> Signed-off-by: Tyrel Datwyler <[email-masked]>
> ---
>  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;
> >  		}