[PATCH v2 3/3] virtiofs: emit uevents on filesystem events

[Stefan Hajnoczi <stefanha@redhat.com>]

Alyssa Ross <hi@alyssa.is> requested that virtiofs notifies userspace
when filesytems become available. This can be used to detect when a
filesystem with a given tag is hotplugged, for example. uevents allow
userspace to detect changes without resorting to polling.

The tag is included as a uevent property so it's easy for userspace to
identify the filesystem in question even when the sysfs directory goes
away during removal.

Here are example uevents:

  # udevadm monitor -k -p

  KERNEL[111.113221] add      /fs/virtiofs/2 (virtiofs)
  ACTION=add
  DEVPATH=/fs/virtiofs/2
  SUBSYSTEM=virtiofs
  TAG=test

  KERNEL[165.527167] remove   /fs/virtiofs/2 (virtiofs)
  ACTION=remove
  DEVPATH=/fs/virtiofs/2
  SUBSYSTEM=virtiofs
  TAG=test

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 fs/fuse/virtio_fs.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
index 28e96b7cde00..18a8f531e5d4 100644
--- a/fs/fuse/virtio_fs.c
+++ b/fs/fuse/virtio_fs.c
@@ -270,6 +270,17 @@ static void virtio_fs_start_all_queues(struct virtio_fs *fs)
 	}
 }
 
+static void virtio_fs_uevent(struct virtio_fs *fs, enum kobject_action action)
+{
+	char tag_str[sizeof("TAG=") +
+		     sizeof_field(struct virtio_fs_config, tag) + 1];
+	char *envp[] = {tag_str, NULL};
+
+	snprintf(tag_str, sizeof(tag_str), "TAG=%s", fs->tag);
+
+	kobject_uevent_env(&fs->kobj, action, envp);
+}
+
 /* Add a new instance to the list or return -EEXIST if tag name exists*/
 static int virtio_fs_add_instance(struct virtio_device *vdev,
 				  struct virtio_fs *fs)
@@ -309,6 +320,8 @@ static int virtio_fs_add_instance(struct virtio_device *vdev,
 
 	mutex_unlock(&virtio_fs_mutex);
 
+	virtio_fs_uevent(fs, KOBJ_ADD);
+
 	return 0;
 }
 
@@ -977,6 +990,8 @@ static void virtio_fs_remove(struct virtio_device *vdev)
 {
 	struct virtio_fs *fs = vdev->priv;
 
+	virtio_fs_uevent(fs, KOBJ_REMOVE);
+
 	mutex_lock(&virtio_fs_mutex);
 	/* This device is going away. No one should get new reference */
 	list_del_init(&fs->list);
-- 
2.43.0