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

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

[Tyrel Datwyler]

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;
 		}
-- 
1.8.3.1

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

[Johannes Thumshirn]

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?

Thanks,
	Johannes
-- 
Johannes Thumshirn                                          Storage
jthumshirn@suse.de                                +49 911 74053 689
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: Felix Imendörffer, Jane Smithard, Graham Norton
HRB 21284 (AG Nürnberg)
Key fingerprint = EC38 9CAB C2C4 F25D 8600 D0D0 0393 969D 2D76 0850

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

[Brian King]

Reviewed-by: Brian King <brking@linux.vnet.ibm.com>

-- 
Brian King
Power Linux I/O
IBM Linux Technology Center

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

[Paolo Bonzini]

On 08/12/2016 00:31, 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();

Should this driver use virt_wmb instead?

Paolo

>  		} else {
>  			done = 1;
>  		}
> 

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

[Tyrel Datwyler]

On 12/08/2016 03:29 PM, Paolo Bonzini wrote:
> 
> 
> On 08/12/2016 00:31, 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();
> 
> Should this driver use virt_wmb instead?

Both virt_wmb and wmb reduce to a lwsync instruction under PowerPC.

-Tyrel

> 
> Paolo
> 
>>  		} else {
>>  			done = 1;
>>  		}
>>

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

[Tyrel Datwyler]

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
> 

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

[Benjamin Herrenschmidt]

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 <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;
> >  		}

Re: [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@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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