iw_cxgb4: don't block in destroy_qp awaiting the last deref

Blocking in c4iw_destroy_qp() causes a deadlock when apps destroy a qp
or disconnect a cm_id from their cm event handler function.  There is
no need to block here anyway, so just replace the refcnt atomic with a
kref object and free the memory on the last put.
Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/hw/cxgb4/iw_cxgb4.h

@@ -475,7 +475,7 @@ struct c4iw_qp {
 	struct t4_wq wq;
 	spinlock_t lock;
 	struct mutex mutex;
-	atomic_t refcnt;
+	struct kref kref;
 	wait_queue_head_t wait;
 	struct timer_list timer;
 	int sq_sig_all;

drivers/infiniband/hw/cxgb4/qp.c

@@ -683,17 +683,25 @@ static int build_inv_stag(union t4_wr *wqe, struct ib_send_wr *wr,
 	return 0;
 }
 
+void _free_qp(struct kref *kref)
+{
+	struct c4iw_qp *qhp;
+
+	qhp = container_of(kref, struct c4iw_qp, kref);
+	PDBG("%s qhp %p\n", __func__, qhp);
+	kfree(qhp);
+}
+
 void c4iw_qp_add_ref(struct ib_qp *qp)
 {
 	PDBG("%s ib_qp %p\n", __func__, qp);
-	atomic_inc(&(to_c4iw_qp(qp)->refcnt));
+	kref_get(&to_c4iw_qp(qp)->kref);
 }
 
 void c4iw_qp_rem_ref(struct ib_qp *qp)
 {
 	PDBG("%s ib_qp %p\n", __func__, qp);
-	if (atomic_dec_and_test(&(to_c4iw_qp(qp)->refcnt)))
-		wake_up(&(to_c4iw_qp(qp)->wait));
+	kref_put(&to_c4iw_qp(qp)->kref, _free_qp);
 }
 
 static void add_to_fc_list(struct list_head *head, struct list_head *entry)
@@ -1594,8 +1602,6 @@ int c4iw_destroy_qp(struct ib_qp *ib_qp)
 	wait_event(qhp->wait, !qhp->ep);
 
 	remove_handle(rhp, &rhp->qpidr, qhp->wq.sq.qid);
-	atomic_dec(&qhp->refcnt);
-	wait_event(qhp->wait, !atomic_read(&qhp->refcnt));
 
 	spin_lock_irq(&rhp->lock);
 	if (!list_empty(&qhp->db_fc_entry))
@@ -1608,8 +1614,9 @@ int c4iw_destroy_qp(struct ib_qp *ib_qp)
 	destroy_qp(&rhp->rdev, &qhp->wq,
 		   ucontext ? &ucontext->uctx : &rhp->rdev.uctx);
 
+	c4iw_qp_rem_ref(ib_qp);
+
 	PDBG("%s ib_qp %p qpid 0x%0x\n", __func__, ib_qp, qhp->wq.sq.qid);
-	kfree(qhp);
 	return 0;
 }
 
@@ -1706,7 +1713,7 @@ struct ib_qp *c4iw_create_qp(struct ib_pd *pd, struct ib_qp_init_attr *attrs,
 	init_completion(&qhp->rq_drained);
 	mutex_init(&qhp->mutex);
 	init_waitqueue_head(&qhp->wait);
-	atomic_set(&qhp->refcnt, 1);
+	kref_init(&qhp->kref);
 
 	ret = insert_handle(rhp, &rhp->qpidr, qhp, qhp->wq.sq.qid);
 	if (ret)