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

Sagi Grimberg
2015-11-15T09:35:02Z

> +
> +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?