block: store the holder in file->private_data

Store the file struct used as the holder in file->private_data as an
indicator that this file descriptor was opened exclusively to  remove
the last use of FMODE_EXCL.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Link: https://lore.kernel.org/r/20230608110258.189493-30-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Christoph Hellwig

block/fops.c

@@ -478,7 +478,7 @@ blk_mode_t file_to_blk_mode(struct file *file)
 		mode |= BLK_OPEN_READ;
 	if (file->f_mode & FMODE_WRITE)
 		mode |= BLK_OPEN_WRITE;
-	if (file->f_mode & FMODE_EXCL)
+	if (file->private_data)
 		mode |= BLK_OPEN_EXCL;
 	if (file->f_flags & O_NDELAY)
 		mode |= BLK_OPEN_NDELAY;
@@ -507,12 +507,15 @@ static int blkdev_open(struct inode *inode, struct file *filp)
 	filp->f_flags |= O_LARGEFILE;
 	filp->f_mode |= FMODE_NOWAIT | FMODE_BUF_RASYNC;
 
+	/*
+	 * Use the file private data to store the holder for exclusive openes.
+	 * file_to_blk_mode relies on it being present to set BLK_OPEN_EXCL.
+	 */
 	if (filp->f_flags & O_EXCL)
-		filp->f_mode |= FMODE_EXCL;
+		filp->private_data = filp;
 
 	bdev = blkdev_get_by_dev(inode->i_rdev, file_to_blk_mode(filp),
-				 (filp->f_mode & FMODE_EXCL) ? filp : NULL,
-				 NULL);
+				 filp->private_data, NULL);
 	if (IS_ERR(bdev))
 		return PTR_ERR(bdev);
 
@@ -523,8 +526,7 @@ static int blkdev_open(struct inode *inode, struct file *filp)
 
 static int blkdev_release(struct inode *inode, struct file *filp)
 {
-	blkdev_put(I_BDEV(filp->f_mapping->host),
-		   (filp->f_mode & FMODE_EXCL) ? filp : NULL);
+	blkdev_put(I_BDEV(filp->f_mapping->host), filp->private_data);
 	return 0;
 }