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

[Juergen Gross <jgross@xxxxxxxx>]

On 29/11/16 12:28, David Vrabel wrote:
> On 29/11/16 11:19, Juergen Gross wrote:
>> 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.
> 
> FWIW, I found the commit message a bit misleading and also came to the
> same conclusion as Jan initially.
> 
> Perhaps,
> 
> "When adding a new request to the ring, an error may cause the
> (partially constructed) request to be discarded and used for the next.
> Thus ring->req_prod_pvt should not be advanced until we know the request
> will be successfully added to the ring."

This is indeed much better, thanks.

In case there are no other objections I'll fix this up when
committing.


Juergen


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