RDMA/irdma: Reduce iWARP QP destroy time

QP destroy is synchronous and waits for its refcnt to be decremented in
irdma_cm_node_free_cb (for iWARP) which fires after the RCU grace period
elapses.

Applications running a large number of connections are exposed to high
wait times on destroy QP for events like SIGABORT.

The long pole for this wait time is the firing of the call_rcu callback
during a CM node destroy which can be slow. It holds the QP reference
count and blocks the destroy QP from completing.

call_rcu only needs to make sure that list walkers have a reference to the
cm_node object before freeing it and thus need to wait for grace period
elapse. The rest of the connection teardown in irdma_cm_node_free_cb is
moved out of the grace period wait in irdma_destroy_connection. Also,
replace call_rcu with a simple kfree_rcu as it just needs to do a kfree on
the cm_node

Fixes: 146b9756f14c ("RDMA/irdma: Add connection manager")
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Shiraz Saleem <[e-mail]>
Signed-off-by: Jason Gunthorpe <[e-mail]>

drivers/infiniband/hw/irdma/cm.c

···
 	return NULL;
 }
 
-static void irdma_cm_node_free_cb(struct rcu_head *rcu_head)
+static void irdma_destroy_connection(struct irdma_cm_node *cm_node)
 {
-	struct irdma_cm_node *cm_node =
-			    container_of(rcu_head, struct irdma_cm_node, rcu_head);
 	struct irdma_cm_core *cm_core = cm_node->cm_core;
 	struct irdma_qp *iwqp;
 	struct irdma_cm_info nfo;
···
 	}
 
 	cm_core->cm_free_ah(cm_node);
-	kfree(cm_node);
 }
 
 /**
···
 
 	spin_unlock_irqrestore(&cm_core->ht_lock, flags);
 
-	/* wait for all list walkers to exit their grace period */
-	call_rcu(&cm_node->rcu_head, irdma_cm_node_free_cb);
+	irdma_destroy_connection(cm_node);
+
+	kfree_rcu(cm_node, rcu_head);
 }
 
 /**