From 7fd152f4b6ae4f3cf89e4b7a0383cc3c470772fc Mon Sep 17 00:00:00 2001
From: Daniel Stodden <daniel.stodden@citrix.com>
Date: Sat, 7 Aug 2010 18:45:12 +0200
Subject: blkfront: Fix blkfront backend switch race (bdev release)

We cannot read backend state within bdev operations, because it risks
grabbing the state change before xenbus gets to do it.

Fixed by tracking deferral with a frontend switch to Closing. State
exposure isn't strictly necessary, but the backends won't mind.

For a 'clean' deferral this seems actually a more decent protocol than
raising errors.

Signed-off-by: Daniel Stodden <daniel.stodden@citrix.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 763a315..4986299 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -1142,31 +1142,48 @@ static int blkif_open(struct block_device *bdev, fmode_t mode)
 	if (!err)
 		++info->users;
 
-	unlock_kernel();
 out:
+	unlock_kernel();
 	return err;
 }
 
 static int blkif_release(struct gendisk *disk, fmode_t mode)
 {
 	struct blkfront_info *info = disk->private_data;
+	struct block_device *bdev;
+	struct xenbus_device *xbdev;
+
 	lock_kernel();
-	info->users--;
-	if (info->users == 0) {
-		/* Check whether we have been instructed to close.  We will
-		   have ignored this request initially, as the device was
-		   still mounted. */
-		struct xenbus_device *dev = info->xbdev;
-
-		if (!dev) {
-			xlvbd_release_gendisk(info);
-			kfree(info);
-		} else if (xenbus_read_driver_state(dev->otherend)
-			   == XenbusStateClosing && info->is_ready) {
-			xlvbd_release_gendisk(info);
-			xenbus_frontend_closed(dev);
-		}
+	if (--info->users)
+		goto out;
+
+	bdev = bdget_disk(disk, 0);
+	bdput(bdev);
+
+	/*
+	 * Check if we have been instructed to close. We will have
+	 * deferred this request, because the bdev was still open.
+	 */
+
+	mutex_lock(&info->mutex);
+	xbdev = info->xbdev;
+
+	if (xbdev && xbdev->state == XenbusStateClosing) {
+		/* pending switch to state closed */
+		xlvbd_release_gendisk(info);
+		xenbus_frontend_closed(info->xbdev);
+ 	}
+
+	mutex_unlock(&info->mutex);
+
+	if (!xbdev) {
+		/* sudden device removal */
+		xlvbd_release_gendisk(info);
+		disk->private_data = NULL;
+		kfree(info);
 	}
+
+out:
 	unlock_kernel();
 	return 0;
 }
-- 
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