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From:   Demi Marie Obenour <demi@invisiblethingslab.com>
To:     Jens Axboe <axboe@kernel.dk>,
        =?UTF-8?q?Roger=20Pau=20Monn=C3=A9?= <roger.pau@citrix.com>,
        Alasdair Kergon <agk@redhat.com>,
        Mike Snitzer <snitzer@kernel.org>, dm-devel@redhat.com
Cc:     Demi Marie Obenour <demi@invisiblethingslab.com>,
        =?UTF-8?q?Marek=20Marczykowski-G=C3=B3recki?= 
        <marmarek@invisiblethingslab.com>, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org
Subject: [PATCH v2 16/16] xen-blkback: Inform userspace that device has been opened
Date:   Tue, 30 May 2023 16:31:16 -0400
Message-Id: <20230530203116.2008-17-demi@invisiblethingslab.com>
In-Reply-To: <20230530203116.2008-1-demi@invisiblethingslab.com>
References: <20230530203116.2008-1-demi@invisiblethingslab.com>
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Set "opened" to "0" before the hotplug script is called.  Once the
device node has been opened, set "opened" to "1".

"opened" is used exclusively by userspace.  It serves two purposes:

1. It tells userspace that the diskseq Xenstore entry 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.

Together, these changes allow userspace to use block devices with
delete-on-close behavior, such as loop devices with the autoclear flag
set or device-mapper devices with the deferred-remove flag set.

Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
---
 drivers/block/xen-blkback/xenbus.c | 35 ++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index 9c3eb148fbd802c74e626c3d7bcd69dcb09bd921..519a78aa9073d1faa1dce5c1b36e95ae58da534b 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -3,6 +3,20 @@
     Copyright (C) 2005 Rusty Russell <rusty@rustcorp.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 read 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 after writing
+   "physical-device".  Once "opened" is 1, blkback has a reference to the
+   device, so userspace doesn't need to keep one.
 
 */
 
@@ -699,6 +713,14 @@ 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;
@@ -826,6 +848,19 @@ 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