RE: [PATCH 3/9] IB: add a helper to safely drain a QP

["Steve Wise" <swise@opengridcomputing.com>]

> -----Original Message-----
> From: Steve Wise [mailto:swise@opengridcomputing.com]
> Sent: Monday, November 16, 2015 10:38 AM
> To: Sagi Grimberg; Christoph Hellwig; linux-rdma@vger.kernel.org
> Cc: bart.vanassche@sandisk.com; axboe@fb.com; linux-scsi@vger.kernel.org; linux-kernel@vger.kernel.org
> Subject: Re: [PATCH 3/9] IB: add a helper to safely drain a QP
> 
> On 11/15/2015 3:34 AM, Sagi Grimberg wrote:
> >
> >> +
> >> +struct ib_stop_cqe {
> >> +    struct ib_cqe    cqe;
> >> +    struct completion done;
> >> +};
> >> +
> >> +static void ib_stop_done(struct ib_cq *cq, struct ib_wc *wc)
> >> +{
> >> +    struct ib_stop_cqe *stop =
> >> +        container_of(wc->wr_cqe, struct ib_stop_cqe, cqe);
> >> +
> >> +    complete(&stop->done);
> >> +}
> >> +
> >> +/*
> >> + * Change a queue pair into the error state and wait until all receive
> >> + * completions have been processed before destroying it. This avoids
> >> that
> >> + * the receive completion handler can access the queue pair while it is
> >> + * being destroyed.
> >> + */
> >> +void ib_drain_qp(struct ib_qp *qp)
> >> +{
> >> +    struct ib_qp_attr attr = { .qp_state = IB_QPS_ERR };
> >> +    struct ib_stop_cqe stop = { };
> >> +    struct ib_recv_wr wr, *bad_wr;
> >> +    int ret;
> >> +
> >> +    wr.wr_cqe = &stop.cqe;
> >> +    stop.cqe.done = ib_stop_done;
> >> +    init_completion(&stop.done);
> >> +
> >> +    ret = ib_modify_qp(qp, &attr, IB_QP_STATE);
> >> +    if (ret) {
> >> +        WARN_ONCE(ret, "failed to drain QP: %d\n", ret);
> >> +        return;
> >> +    }
> >> +
> >> +    ret = ib_post_recv(qp, &wr, &bad_wr);
> >> +    if (ret) {
> >> +        WARN_ONCE(ret, "failed to drain QP: %d\n", ret);
> >> +        return;
> >> +    }
> >> +
> >> +    wait_for_completion(&stop.done);
> >> +}
> >
> > This is taken from srp, and srp drains using a recv wr due to a race
> > causing a use-after-free condition in srp which re-posts a recv buffer
> > in the recv completion handler. srp does not really care if there are
> > pending send flushes.
> >
> > I'm not sure if there are ordering rules for send/recv queues in
> > terms of flush completions, meaning that even if all recv flushes
> > were consumed maybe there are send flushes still pending.
> >
> > I think that for a general drain helper it would be useful to
> > make sure that both the recv _and_ send flushes were drained.
> >
> > So, something like:
> >
> > void ib_drain_qp(struct ib_qp *qp)
> > {
> >     struct ib_qp_attr attr = { .qp_state = IB_QPS_ERR };
> >     struct ib_stop_cqe rstop, sstop;
> >     struct ib_recv_wr rwr = {}, *bad_rwr;
> >     struct ib_send_wr swr = {}, *bad_swr;
> >     int ret;
> >
> >     rwr.wr_cqe = &rstop.cqe;
> >     rstop.cqe.done = ib_stop_done;
> >     init_completion(&rstop.done);
> >
> >     swr.wr_cqe = &sstop.cqe;
> >     sstop.cqe.done = ib_stop_done;
> >     init_completion(&sstop.done);
> >
> >     ret = ib_modify_qp(qp, &attr, IB_QP_STATE);
> >     if (ret) {
> >         WARN_ONCE(ret, "failed to drain QP: %d\n", ret);
> >         return;
> >     }
> >
> >     ret = ib_post_recv(qp, &rwr, &bad_rwr);
> >     if (ret) {
> >         WARN_ONCE(ret, "failed to drain recv queue: %d\n", ret);
> >         return;
> >     }
> >
> >     ret = ib_post_send(qp, &swr, &bad_swr);
> >     if (ret) {
> >         WARN_ONCE(ret, "failed to drain send queue: %d\n", ret);
> >         return;
> >     }
> >
> >     wait_for_completion(&rstop.done);
> >     wait_for_completion(&sstop.done);
> > }
> >
> > Thoughts?
> 
> This won't work for iWARP as per my previous email.  But I will code
> something up that will.
> 
> Steve

After looking at the nes driver, I don't see any common way to support drain w/o some serious driver mods.  Since SRP is the only
user, perhaps we can ignore iWARP for this function...