virtio: console: Add reference counting for port struct

When a port got hot-unplugged, when a port was open, any file operation
after the unplugging resulted in a crash. This is fixed by ref-counting
the port structure, and releasing it only when the file is closed.

This splits the unplug operation in two parts: first marks the port
as unavailable, removes all the buffers in the vqs and removes the port
from the per-device list of ports. The second stage, invoked when all
references drop to zero, releases the chardev and frees all other memory.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index 8a9c140..288701c 100644 (file)
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -187,6 +187,9 @@ struct port {
        struct cdev *cdev;
        struct device *dev;
 
+       /* Reference-counting to handle port hot-unplugs and file operations */
+       struct kref kref;
+
        /* A waitqueue for poll() or blocking read operations */
        wait_queue_head_t waitqueue;
 
@@ -725,6 +728,8 @@ static unsigned int port_fops_poll(struct file *filp, poll_table *wait)
        return ret;
 }
 
+static void remove_port(struct kref *kref);
+
 static int port_fops_release(struct inode *inode, struct file *filp)
 {
        struct port *port;
@@ -745,6 +750,16 @@ static int port_fops_release(struct inode *inode, struct file *filp)
        reclaim_consumed_buffers(port);
        spin_unlock_irq(&port->outvq_lock);
 
+       /*
+        * Locks aren't necessary here as a port can't be opened after
+        * unplug, and if a port isn't unplugged, a kref would already
+        * exist for the port.  Plus, taking ports_lock here would
+        * create a dependency on other locks taken by functions
+        * inside remove_port if we're the last holder of the port,
+        * creating many problems.
+        */
+       kref_put(&port->kref, remove_port);
+
        return 0;
 }
 
@@ -757,6 +772,11 @@ static int port_fops_open(struct inode *inode, struct file *filp)
        port = find_port_by_devt(cdev->dev);
        filp->private_data = port;
 
+       /* Prevent against a port getting hot-unplugged at the same time */
+       spin_lock_irq(&port->portdev->ports_lock);
+       kref_get(&port->kref);
+       spin_unlock_irq(&port->portdev->ports_lock);
+
        /*
         * Don't allow opening of console port devices -- that's done
         * via /dev/hvc
@@ -791,6 +811,7 @@ static int port_fops_open(struct inode *inode, struct file *filp)
 
        return 0;
 out:
+       kref_put(&port->kref, remove_port);
        return ret;
 }
 
@@ -1079,6 +1100,7 @@ static int add_port(struct ports_device *portdev, u32 id)
                err = -ENOMEM;
                goto fail;
        }
+       kref_init(&port->kref);
 
        port->portdev = portdev;
        port->id = id;
@@ -1183,22 +1205,43 @@ fail:
        return err;
 }
 
-/* Remove all port-specific data. */
-static void remove_port(struct port *port)
+/* No users remain, remove all port-specific data. */
+static void remove_port(struct kref *kref)
+{
+       struct port *port;
+
+       port = container_of(kref, struct port, kref);
+
+       sysfs_remove_group(&port->dev->kobj, &port_attribute_group);
+       device_destroy(pdrvdata.class, port->dev->devt);
+       cdev_del(port->cdev);
+
+       kfree(port->name);
+
+       debugfs_remove(port->debugfs_file);
+
+       kfree(port);
+}
+
+/*
+ * Port got unplugged.  Remove port from portdev's list and drop the
+ * kref reference.  If no userspace has this port opened, it will
+ * result in immediate removal the port.
+ */
+static void unplug_port(struct port *port)
 {
        struct port_buffer *buf;
 
+       spin_lock_irq(&port->portdev->ports_lock);
+       list_del(&port->list);
+       spin_unlock_irq(&port->portdev->ports_lock);
+
        if (port->guest_connected) {
                port->guest_connected = false;
                port->host_connected = false;
                wake_up_interruptible(&port->waitqueue);
-               send_control_msg(port, VIRTIO_CONSOLE_PORT_OPEN, 0);
        }
 
-       spin_lock_irq(&port->portdev->ports_lock);
-       list_del(&port->list);
-       spin_unlock_irq(&port->portdev->ports_lock);
-
        if (is_console_port(port)) {
                spin_lock_irq(&pdrvdata_lock);
                list_del(&port->cons.list);
@@ -1216,9 +1259,6 @@ static void remove_port(struct port *port)
                hvc_remove(port->cons.hvc);
 #endif
        }
-       sysfs_remove_group(&port->dev->kobj, &port_attribute_group);
-       device_destroy(pdrvdata.class, port->dev->devt);
-       cdev_del(port->cdev);
 
        /* Remove unused data this port might have received. */
        discard_port_data(port);
@@ -1229,11 +1269,19 @@ static void remove_port(struct port *port)
        while ((buf = virtqueue_detach_unused_buf(port->in_vq)))
                free_buf(buf);
 
-       kfree(port->name);
-
-       debugfs_remove(port->debugfs_file);
+       /*
+        * We should just assume the device itself has gone off --
+        * else a close on an open port later will try to send out a
+        * control message.
+        */
+       port->portdev = NULL;
 
-       kfree(port);
+       /*
+        * Locks around here are not necessary - a port can't be
+        * opened after we removed the port struct from ports_list
+        * above.
+        */
+       kref_put(&port->kref, remove_port);
 }
 
 /* Any private messages that the Host and Guest want to share */
@@ -1272,7 +1320,7 @@ static void handle_control_message(struct ports_device *portdev,
                add_port(portdev, cpkt->id);
                break;
        case VIRTIO_CONSOLE_PORT_REMOVE:
-               remove_port(port);
+               unplug_port(port);
                break;
        case VIRTIO_CONSOLE_CONSOLE_PORT:
                if (!cpkt->value)
@@ -1686,7 +1734,7 @@ static void virtcons_remove(struct virtio_device *vdev)
        cancel_work_sync(&portdev->control_work);
 
        list_for_each_entry_safe(port, port2, &portdev->ports, list)
-               remove_port(port);
+               unplug_port(port);
 
        unregister_chrdev(portdev->chr_major, "virtio-portsdev");