[RFC PATCH 7/7] xen/blkback: Inform userspace that device has been opened

[Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>]

This allows userspace to use block devices with delete-on-close
behavior, which is necessary to ensure virtual devices (such as loop or
device-mapper devices) are cleaned up automatically.  Protocol details
are included in comments.

Signed-off-by: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>
---
 drivers/block/xen-blkback/xenbus.c | 34 ++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/drivers/block/xen-blkback/xenbus.c 
b/drivers/block/xen-blkback/xenbus.c
index 
2c43bfc7ab5ba6954f11d4b949a5668660dbd290..ca8dae05985038da490c5ac93364509913f6b4c7
 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -3,6 +3,19 @@
     Copyright (C) 2005 Rusty Russell <rusty@xxxxxxxxxxxxxxx>
     Copyright (C) 2005 XenSource Ltd
 
+In addition to the XenStore nodes required by the Xen block device
+specification, this implementation of blkback uses a new XenStore
+node: "opened".  blkback sets "opened" to "0" before the hotplug script
+is called.  Once the device node has been opened, blkback sets "opened"
+to "1".
+
+"opened" is used exclusively by userspace.  It serves two purposes:
+
+1. It tells userspace that diskseq@major:minor syntax for "physical-device" is
+   supported.
+2. It tells userspace that it can wait for "opened" to be set to 1.  Once
+   "opened" is 1, blkback has a reference to the device, so userspace doesn't
+   need to keep one.
 
 */
 
@@ -698,6 +711,14 @@ static int xen_blkbk_probe(struct xenbus_device *dev,
        if (err)
                pr_warn("%s write out 'max-ring-page-order' failed\n", 
__func__);
 
+       /*
+        * This informs userspace that the "opened" node will be set to "1" when
+        * the device has been opened successfully.
+        */
+       err = xenbus_write(XBT_NIL, dev->nodename, "opened", "0");
+       if (err)
+               goto fail;
+
        err = xenbus_switch_state(dev, XenbusStateInitWait);
        if (err)
                goto fail;
@@ -824,6 +845,19 @@ static void backend_changed(struct xenbus_watch *watch,
                goto fail;
        }
 
+       /*
+        * Tell userspace that the device has been opened and that blkback has a
+        * reference to it.  Userspace can then close the device or mark it as
+        * delete-on-close, knowing that blkback will keep the device open as
+        * long as necessary.
+        */
+       err = xenbus_write(XBT_NIL, dev->nodename, "opened", "1");
+       if (err) {
+               xenbus_dev_fatal(dev, err, "%s: notifying userspace device has 
been opened",
+                                dev->nodename);
+               goto free_vbd;
+       }
+
        err = xenvbd_sysfs_addif(dev);
        if (err) {
                xenbus_dev_fatal(dev, err, "creating sysfs entries");
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab