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Date: Tue, 29 Nov 2016 11:28:39 +0000 From: David Vrabel <david.vrabel@...rix.com> To: Juergen Gross <jgross@...e.com>, Jan Beulich <JBeulich@...e.com> CC: <jejb@...ux.vnet.ibm.com>, <linux-scsi@...r.kernel.org>, <martin.petersen@...cle.com>, <lambert.quentin@...il.com>, <linux-kernel@...r.kernel.org>, <xen-devel@...ts.xenproject.org>, <boris.ostrovsky@...cle.com>, <dan.carpenter@...cle.com> Subject: Re: [Xen-devel] [PATCH] xen/scsifront: don't advance ring request pointer in case of error 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@...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. 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." Or similar. David
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