RDS/IB: add refcount tracking to struct rds_ib_device

The RDS IB client .remove callback used to free the rds_ibdev for the given
device unconditionally.  This could race other users of the struct.  This patch
adds refcounting so that we only free the rds_ibdev once all of its users are
done.

Many rds_ibdev users are tied to connections.  We give the connection a
reference and change these users to reference the device in the connection
instead of looking it up in the IB client data.  The only user of the IB client
data remaining is the first lookup of the device as connections are built up.

Incrementing the reference count of a device found in the IB client data could
race with final freeing so we use an RCU grace period to make sure that freeing
won't happen until those lookups are done.

MRs need the rds_ibdev to get at the pool that they're freed in to.  They exist
outside a connection and many MRs can reference different devices from one
socket, so it was natural to have each MR hold a reference.  MR refs can be
dropped from interrupt handlers and final device teardown can block so we push
it off to a work struct.  Pool teardown had to be fixed to cancel its pending
work instead of deadlocking waiting for all queued work, including itself, to
finish.

MRs get their reference from the global device list, which gets a reference.
It is left unprotected by locks and remains racy.  A simple global lock would
be a significant bottleneck.  More scalable (complicated) locking should be
done carefully in a later patch.

Signed-off-by: Zach Brown <zach.brown@oracle.com>

1 files changed, 95 insertions, 34 deletions

diff --git a/net/rds/ib.c b/net/rds/ib.c
index 7d289d7985fe..1732f8effb59 100644
--- a/net/rds/ib.c
+++ b/net/rds/ib.c
@@ -59,6 +59,38 @@ struct list_head rds_ib_devices;
 DEFINE_SPINLOCK(ib_nodev_conns_lock);
 LIST_HEAD(ib_nodev_conns);
 
+/*
+ * rds_ib_destroy_mr_pool() blocks on a few things and mrs drop references
+ * from interrupt context so we push freing off into a work struct in krdsd.
+ */
+static void rds_ib_dev_free(struct work_struct *work)
+{
+        struct rds_ib_ipaddr *i_ipaddr, *i_next;
+        struct rds_ib_device *rds_ibdev = container_of(work,
+                                        struct rds_ib_device, free_work);
+
+        if (rds_ibdev->mr_pool)
+                rds_ib_destroy_mr_pool(rds_ibdev->mr_pool);
+        if (rds_ibdev->mr)
+                ib_dereg_mr(rds_ibdev->mr);
+        if (rds_ibdev->pd)
+                ib_dealloc_pd(rds_ibdev->pd);
+
+        list_for_each_entry_safe(i_ipaddr, i_next, &rds_ibdev->ipaddr_list, list) {
+                list_del(&i_ipaddr->list);
+                kfree(i_ipaddr);
+        }
+
+        kfree(rds_ibdev);
+}
+
+void rds_ib_dev_put(struct rds_ib_device *rds_ibdev)
+{
+        BUG_ON(atomic_read(&rds_ibdev->refcount) <= 0);
+        if (atomic_dec_and_test(&rds_ibdev->refcount))
+                queue_work(rds_wq, &rds_ibdev->free_work);
+}
+
 void rds_ib_add_one(struct ib_device *device)
 {
         struct rds_ib_device *rds_ibdev;
@@ -77,11 +109,14 @@ void rds_ib_add_one(struct ib_device *device)
                 goto free_attr;
         }
 
-        rds_ibdev = kmalloc_node(sizeof *rds_ibdev, GFP_KERNEL, ibdev_to_node(device));
+        rds_ibdev = kzalloc_node(sizeof(struct rds_ib_device), GFP_KERNEL,
+                                 ibdev_to_node(device));
         if (!rds_ibdev)
                 goto free_attr;
 
         spin_lock_init(&rds_ibdev->spinlock);
+        atomic_set(&rds_ibdev->refcount, 1);
+        INIT_WORK(&rds_ibdev->free_work, rds_ib_dev_free);
 
         rds_ibdev->max_wrs = dev_attr->max_qp_wr;
         rds_ibdev->max_sge = min(dev_attr->max_sge, RDS_IB_MAX_SGE);
@@ -96,67 +131,93 @@ void rds_ib_add_one(struct ib_device *device)
 
         rds_ibdev->dev = device;
         rds_ibdev->pd = ib_alloc_pd(device);
-        if (IS_ERR(rds_ibdev->pd))
-                goto free_dev;
+        if (IS_ERR(rds_ibdev->pd)) {
+                rds_ibdev->pd = NULL;
+                goto put_dev;
+        }
 
-        rds_ibdev->mr = ib_get_dma_mr(rds_ibdev->pd,
-                                      IB_ACCESS_LOCAL_WRITE);
-        if (IS_ERR(rds_ibdev->mr))
-                goto err_pd;
+        rds_ibdev->mr = ib_get_dma_mr(rds_ibdev->pd, IB_ACCESS_LOCAL_WRITE);
+        if (IS_ERR(rds_ibdev->mr)) {
+                rds_ibdev->mr = NULL;
+                goto put_dev;
+        }
 
         rds_ibdev->mr_pool = rds_ib_create_mr_pool(rds_ibdev);
         if (IS_ERR(rds_ibdev->mr_pool)) {
                 rds_ibdev->mr_pool = NULL;
-                goto err_mr;
+                goto put_dev;
         }
 
         INIT_LIST_HEAD(&rds_ibdev->ipaddr_list);
         INIT_LIST_HEAD(&rds_ibdev->conn_list);
         list_add_tail(&rds_ibdev->list, &rds_ib_devices);
+        atomic_inc(&rds_ibdev->refcount);
 
         ib_set_client_data(device, &rds_ib_client, rds_ibdev);
+        atomic_inc(&rds_ibdev->refcount);
 
-        goto free_attr;
-
-err_mr:
-        ib_dereg_mr(rds_ibdev->mr);
-err_pd:
-        ib_dealloc_pd(rds_ibdev->pd);
-free_dev:
-        kfree(rds_ibdev);
+put_dev:
+        rds_ib_dev_put(rds_ibdev);
 free_attr:
         kfree(dev_attr);
 }
 
+/*
+ * New connections use this to find the device to associate with the
+ * connection.  It's not in the fast path so we're not concerned about the
+ * performance of the IB call.  (As of this writing, it uses an interrupt
+ * blocking spinlock to serialize walking a per-device list of all registered
+ * clients.)
+ *
+ * RCU is used to handle incoming connections racing with device teardown.
+ * Rather than use a lock to serialize removal from the client_data and
+ * getting a new reference, we use an RCU grace period.  The destruction
+ * path removes the device from client_data and then waits for all RCU
+ * readers to finish.
+ *
+ * A new connection can get NULL from this if its arriving on a
+ * device that is in the process of being removed.
+ */
+struct rds_ib_device *rds_ib_get_client_data(struct ib_device *device)
+{
+        struct rds_ib_device *rds_ibdev;
+
+        rcu_read_lock();
+        rds_ibdev = ib_get_client_data(device, &rds_ib_client);
+        if (rds_ibdev)
+                atomic_inc(&rds_ibdev->refcount);
+        rcu_read_unlock();
+        return rds_ibdev;
+}
+
+/*
+ * The IB stack is letting us know that a device is going away.  This can
+ * happen if the underlying HCA driver is removed or if PCI hotplug is removing
+ * the pci function, for example.
+ *
+ * This can be called at any time and can be racing with any other RDS path.
+ */
 void rds_ib_remove_one(struct ib_device *device)
 {
         struct rds_ib_device *rds_ibdev;
-        struct rds_ib_ipaddr *i_ipaddr, *i_next;
 
         rds_ibdev = ib_get_client_data(device, &rds_ib_client);
         if (!rds_ibdev)
                 return;
 
-        synchronize_rcu();
-        list_for_each_entry_safe(i_ipaddr, i_next, &rds_ibdev->ipaddr_list, list) {
-                list_del(&i_ipaddr->list);
-                kfree(i_ipaddr);
-        }
-
         rds_ib_destroy_conns(rds_ibdev);
 
-        if (rds_ibdev->mr_pool)
-                rds_ib_destroy_mr_pool(rds_ibdev->mr_pool);
-
-        ib_dereg_mr(rds_ibdev->mr);
-
-        while (ib_dealloc_pd(rds_ibdev->pd)) {
-                rdsdebug("Failed to dealloc pd %p\n", rds_ibdev->pd);
-                msleep(1);
-        }
+        /*
+         * prevent future connection attempts from getting a reference to this
+         * device and wait for currently racing connection attempts to finish
+         * getting their reference
+         */
+        ib_set_client_data(device, &rds_ib_client, NULL);
+        synchronize_rcu();
+        rds_ib_dev_put(rds_ibdev);
 
         list_del(&rds_ibdev->list);
-        kfree(rds_ibdev);
+        rds_ib_dev_put(rds_ibdev);
 }
 
 struct ib_client rds_ib_client = {
@@ -190,7 +251,7 @@ static int rds_ib_conn_info_visitor(struct rds_connection *conn,
                 rdma_addr_get_sgid(dev_addr, (union ib_gid *) &iinfo->src_gid);
                 rdma_addr_get_dgid(dev_addr, (union ib_gid *) &iinfo->dst_gid);
 
-                rds_ibdev = ib_get_client_data(ic->i_cm_id->device, &rds_ib_client);
+                rds_ibdev = ic->rds_ibdev;
                 iinfo->max_send_wr = ic->i_send_ring.w_nr;
                 iinfo->max_recv_wr = ic->i_recv_ring.w_nr;
                 iinfo->max_send_sge = rds_ibdev->max_sge;