PM / Hibernate: Replace bdget call with simple atomic_inc of i_count

Create bdgrab().  This function copies an existing reference to a
block_device.  It is safe to call from any context.

Hibernation code wishes to copy a reference to the active swap device.
Right now it calls bdget() under a spinlock, but this is wrong because
bdget() can sleep.  It doesn't need a full bdget() because we already
hold a reference to active swap devices (and the spinlock protects
against swapoff).

Fixes http://bugzilla.kernel.org/show_bug.cgi?id=13827

Signed-off-by: Alan Jenkins <alan-jenkins@tuffmail.co.uk>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>

fs/block_dev.c

@@ -564,6 +564,16 @@ struct block_device *bdget(dev_t dev)
 
 EXPORT_SYMBOL(bdget);
 
+/**
+ * bdgrab -- Grab a reference to an already referenced block device
+ * @bdev:	Block device to grab a reference to.
+ */
+struct block_device *bdgrab(struct block_device *bdev)
+{
+	atomic_inc(&bdev->bd_inode->i_count);
+	return bdev;
+}
+
 long nr_blockdev_pages(void)
 {
 	struct block_device *bdev;

include/linux/fs.h

@@ -1946,6 +1946,7 @@ extern void putname(const char *name);
 extern int register_blkdev(unsigned int, const char *);
 extern void unregister_blkdev(unsigned int, const char *);
 extern struct block_device *bdget(dev_t);
+extern struct block_device *bdgrab(struct block_device *bdev);
 extern void bd_set_size(struct block_device *, loff_t size);
 extern void bd_forget(struct inode *inode);
 extern void bdput(struct block_device *);

mm/swapfile.c

@@ -753,7 +753,7 @@ int swap_type_of(dev_t device, sector_t offset, struct block_device **bdev_p)
 
 		if (!bdev) {
 			if (bdev_p)
-				*bdev_p = bdget(sis->bdev->bd_dev);
+				*bdev_p = bdgrab(sis->bdev);
 
 			spin_unlock(&swap_lock);
 			return i;
@@ -765,7 +765,7 @@ int swap_type_of(dev_t device, sector_t offset, struct block_device **bdev_p)
 					struct swap_extent, list);
 			if (se->start_block == offset) {
 				if (bdev_p)
-					*bdev_p = bdget(sis->bdev->bd_dev);
+					*bdev_p = bdgrab(sis->bdev);
 
 				spin_unlock(&swap_lock);
 				bdput(bdev);