# HG changeset patch
# User Daniel Stodden <daniel.stodden@xxxxxxxxxx>
# Date 1272506749 25200
# Node ID b324514f8f83ef0d8c7991e2c995400937c540ac
# Parent  5e73c8d7aaf5bff5caa7d431184ffed8e837997a
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@xxxxxxxxxx>

diff -r 5e73c8d7aaf5 -r b324514f8f83 drivers/block/xen-blkfront.c
--- a/drivers/block/xen-blkfront.c	Wed Apr 28 19:05:49 2010 -0700
+++ b/drivers/block/xen-blkfront.c	Wed Apr 28 19:05:49 2010 -0700
@@ -1122,22 +1122,37 @@
 static int blkif_release(struct gendisk *disk, fmode_t mode)
 {
 	struct blkfront_info *info = disk->private_data;
-	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;
+	struct block_device *bdev;
+	struct xenbus_device *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)
+		return 0;
+
+	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);
+		kfree(info);
+	}
+
 	return 0;
 }