dm: noflush resizing (2/3) Use bdlookup() on noflush suspend

This patch makes device-mapper to use bdlookup to allow resizing of noflush-suspended device.

With this patch, by using bdlookup() instead of bdget(), resizing will no longer wait for I_LOCK.

There is a race between bdlookup() and bdget(). However, it's benign because the only use of bdev obtained by bdlookup() is to modify i_size and bdget() will anyway see the updated disk size.
(See below)

Case 1:
CPU 0 CPU 1
------------------------------------------------------------------
set_capacity()
disk size is updated
bdlookup() -> NULL
bdget()
bdget reads updated disk size and set it to bdev inode's i_size

Case 2:
CPU 0 CPU 1
------------------------------------------------------------------
set_capacity()
disk size is updated
bdget()
bdget reads updated disk size and set it to bdev inode's i_size
bdlookup()
wait for the bdget completion (I_NEW)
and get the bdev inode with updated disk size

Case 3:
CPU 0 CPU 1
------------------------------------------------------------------
bdget()
bdget reads old disk size
set_capacity()
disk size is updated
bdlookup()
get the existing bdev inode
update the i_size

To change i_size of bdev inode, the new code uses bd_mutex instead of inode->i_mutex. According to a comment in include/linux/fs.h, mutex is necessary here only to serialize i_size_write().
bd_inode->i_size is set via bd_set_size() and bd_set_size() is protected by bd_mutex.
So I think bd_mutex is the lock we should use.

---
 dm.c |   27 ++++++++++++++++++++++-----
 1 file changed, 22 insertions(+), 5 deletions(-)

Index: linux-2.6.23.work/drivers/md/dm.c
===================================================================
@@ -1101,11 +1101,29 @@ static void event_callback(void *context
 
 static void __set_size(struct mapped_device *md, sector_t size)
 {
+	struct block_device *bdev;
+
 	set_capacity(md->disk, size);
 
-	mutex_lock(&md->suspended_bdev->bd_inode->i_mutex);
-	i_size_write(md->suspended_bdev->bd_inode, (loff_t)size << SECTOR_SHIFT);
-	mutex_unlock(&md->suspended_bdev->bd_inode->i_mutex);
+	/*
+	 * Updated disk size should be visible to bdget() possibly
+	 * called in parallel to bdlookup()
+	 */
+	smp_mb();
+
+	if (md->suspended_bdev)
+		bdev = md->suspended_bdev;
+	else
+		bdev = bdlookup_disk(md->disk, 0);
+
+	if (bdev) {
+		mutex_lock(&bdev->bd_mutex);
+		i_size_write(bdev->bd_inode, (loff_t)size << SECTOR_SHIFT);
+		mutex_unlock(&bdev->bd_mutex);
+	}
+
+	if (!md->suspended_bdev)
+		bdput(bdev);
 }
 
 static int __bind(struct mapped_device *md, struct dm_table *t)

@@ -1121,8 +1139,7 @@ static int __bind(struct mapped_device *
 	if (size != get_capacity(md->disk))
 		memset(&md->geometry, 0, sizeof(md->geometry));
 
-	if (md->suspended_bdev)
-		__set_size(md, size);
+	__set_size(md, size);
 	if (size == 0)
 		return 0;