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

From: Johannes Thumshirn <[email-masked]>
Date: Thu, 8 Dec 2016 10:06:43 +0100

On Wed, Dec 07, 2016 at 05:31:26PM -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.
> 
> 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;
>  		}

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

Thanks,
	Johannes
-- 
Johannes Thumshirn                                          Storage
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