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

[Tyrel Datwyler <tyreld@linux.vnet.ibm.com>]

On 12/08/2016 01:06 AM, Johannes Thumshirn wrote:
> 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 <tyreld@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;
>>  		}
> 
> 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
>