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

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Date:   Tue, 29 Nov 2016 12:19:33 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Jan Beulich <JBeulich@...e.com>
Cc:     lambert.quentin@...il.com, jejb@...ux.vnet.ibm.com,
        xen-devel@...ts.xenproject.org, boris.ostrovsky@...cle.com,
        dan.carpenter@...cle.com, martin.petersen@...cle.com,
        linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org
Subject: Re: [Xen-devel] [PATCH] xen/scsifront: don't advance ring request
 pointer in case of error

On 29/11/16 12:14, Jan Beulich wrote:
>>>> On 29.11.16 at 11:50, <JGross@...e.com> 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