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

[Tyrel Datwyler]

On 12/09/2016 01:20 PM, Benjamin Herrenschmidt wrote:
> 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.

So, this patch initially came about while chasing a data integrity issue
based on the observation that we were already doing this same write
barrier in the virtual fibre channel driver.

However, the more I stare at it I agree it does seem fishy and I can't
see any other stores that we need to order with here. In terms of the
16byte CRQ entries we only ever write to the first byte as a sort of
doorbell to tell the VIOS that we have processed the data and the entry
is free to be re-used. The remainder of the CRQ is only ever read from.

The wmb() in the ibmvfc driver was part of a commit from Brian that also
involved a necessary rmb() that prevented stale data from load
re-ordering by ensuring that a read from the first byte completed and
contained a valid value prior to doing any other reads of the CRQ entry.

I'd have to defer to Brian as to whether he remembers a legitimate
reason for the wmb().

-Tyrel

> 
> Cheers,
> Ben.
> 
>> Signed-off-by: Tyrel Datwyler <tyreld at linux.vnet.ibm.com>
>> ---
>>  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;
>>>  		}
> 
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