[PATCH] When a partition is claimed, claim the whole device

From: NeilBrown <neilb@cse.unsw.edu.au>

Current devices can be 'claimed' by filesystems (when mounting) or
md/raid (when being included in an array) or 'raw' or ....
This stop concurrent access by these systems.

However it is still possible for one system to claim the whole device
and a second system to claim one partition, which is not good.

With this patch, when a partition is claimed, the whole device is 
claimed for partitioning.  So you cannot have a partition and the
whole devices claimed at the same time (except if the whole device
is claimed for partitioning).

fs/block_dev.c

@@ -419,12 +419,34 @@ void bd_forget(struct inode *inode)
 
 int bd_claim(struct block_device *bdev, void *holder)
 {
-	int res = -EBUSY;
+	int res;
 	spin_lock(&bdev_lock);
-	if (!bdev->bd_holder || bdev->bd_holder == holder) {
-		bdev->bd_holder = holder;
+
+	/* first decide result */
+	if (bdev->bd_holder == holder)
+		res = 0;	 /* already a holder */
+	else if (bdev->bd_holder != NULL)
+		res = -EBUSY; 	 /* held by someone else */
+	else if (bdev->bd_contains == bdev)
+		res = 0;  	 /* is a whole device which isn't held */
+
+	else if (bdev->bd_contains->bd_holder == bd_claim)
+		res = 0; 	 /* is a partition of a device that is being partitioned */
+	else if (bdev->bd_contains->bd_holder != NULL)
+		res = -EBUSY;	 /* is a partition of a held device */
+	else
+		res = 0;	 /* is a partition of an un-held device */
+
+	/* now impose change */
+	if (res==0) {
+		/* note that for a whole device bd_holders
+		 * will be incremented twice, and bd_holder will
+		 * be set to bd_claim before being set to holder
+		 */
+		bdev->bd_contains->bd_holders ++;
+		bdev->bd_contains->bd_holder = bd_claim;
 		bdev->bd_holders++;
-		res = 0;
+		bdev->bd_holder = holder;
 	}
 	spin_unlock(&bdev_lock);
 	return res;
@@ -433,6 +455,8 @@ int bd_claim(struct block_device *bdev, void *holder)
 void bd_release(struct block_device *bdev)
 {
 	spin_lock(&bdev_lock);
+	if (!--bdev->bd_contains->bd_holders)
+		bdev->bd_contains->bd_holder = NULL;
 	if (!--bdev->bd_holders)
 		bdev->bd_holder = NULL;
 	spin_unlock(&bdev_lock);