Re: [Xen-devel] [PATCH] xen/scsifront: don't advance ring request pointer in case of error

[Juergen Gross]

On 29/11/16 12:14, Jan Beulich wrote:
>>>> On 29.11.16 at 11:50, <JGross@xxxxxxxx> wrote:
>> --- a/drivers/scsi/xen-scsifront.c
>> +++ b/drivers/scsi/xen-scsifront.c
>> @@ -184,8 +184,6 @@ static struct vscsiif_request *scsifront_pre_req(struct vscsifrnt_info *info)
>>  
>>  	ring_req = RING_GET_REQUEST(&(info->ring), ring->req_prod_pvt);
>>  
>> -	ring->req_prod_pvt++;
> 
> Please note the "_pvt" suffix, which stands for "private": This field is
> not visible to the backend. Only ring->sring fields are shared, and
> the updating of the shared field happens in RING_PUSH_REQUESTS()
> and RING_PUSH_REQUESTS_AND_CHECK_NOTIFY().

Sure, but RING_PUSH_REQUESTS() will copy req_prod_pvt to req_prod. In
the case corrected this would advance req_prod by two after the error
case before, even if only one request would have made it to the ring.

As an alternative I could have decremented req_prod_pvt in case of an
error, but I like my current solution better.


Juergen