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

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Date:   Wed, 25 Jan 2023 22:33:58 -0500
From:   Demi Marie Obenour <demi@...isiblethingslab.com>
To:     Jens Axboe <axboe@...nel.dk>,
        Roger Pau Monné <roger.pau@...rix.com>,
        Juergen Gross <jgross@...e.com>
Cc:     Demi Marie Obenour <demi@...isiblethingslab.com>,
        Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>, xen-devel@...ts.xenproject.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [RFC PATCH 7/7] xen/blkback: Inform userspace that device has been opened

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@...isiblethingslab.com>
---
 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@...tcorp.com.au>
     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@...or: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