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

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+
+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?
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