[PATCH rfc] nvme-pci: make sure to flush sqe writes before db record update

[jgg@ziepe.ca (Jason Gunthorpe)]

On Thu, Mar 08, 2018@08:51:47AM -0700, Keith Busch wrote:
> On Wed, Mar 07, 2018@07:56:26PM +0200, Sagi Grimberg wrote:
> > @@ -437,8 +437,14 @@ static void __nvme_submit_cmd(struct nvme_queue *nvmeq,
> >  	if (++tail == nvmeq->q_depth)
> >  		tail = 0;
> >  	if (nvme_dbbuf_update_and_check_event(tail, nvmeq->dbbuf_sq_db,
> > -					      nvmeq->dbbuf_sq_ei))
> > +					      nvmeq->dbbuf_sq_ei)) {
> > +		/*
> > +		 * Make sure that descriptors are written before
> > +		 * doorbell record.
> > +		 */
> > +		wmb();
> >  		writel(tail, nvmeq->q_db);
> > +	}
> >  	nvmeq->sq_tail = tail;
> >  }
> 
> If this really is necessary, we'd need this before updating the event
> shadow registers too.
> 
> I'd like to understand this a bit more as we haven't done this in eight
> years and I can't recall any issues around this section. Have we just
> been fortunate that the problem this fixes is extraordinarily unlikely,
> or is something else implicitly ordering within this critical section?

Well, there is a wmb() already inside
nvme_dbbuf_update_and_check_event so any failure would only
be related to dbbuf_sq_db being wrong when tail is written to q_db.

Guessing that might be a basically undetectable situation, maybe some
temporary higher latency or something?

Jason