bdev: infrastructure for flags

Replace bd_partno with a 32bit field (__bd_flags).  The lower 8 bits
contain the partition number, the upper 24 are for flags.

Helpers: bdev_{test,set,clear}_flag(bdev, flag), with atomic_or()
and atomic_andnot() used to set/clear.

NOTE: this commit does not actually move any flags over there - they
are still bool fields.  As the result, it shifts the fields wrt
cacheline boundaries; that's going to be restored once the first
3 flags are dealt with.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

block/bdev.c

@@ -411,7 +411,7 @@ struct block_device *bdev_alloc(struct gendisk *disk, u8 partno)
 	mutex_init(&bdev->bd_fsfreeze_mutex);
 	spin_lock_init(&bdev->bd_size_lock);
 	mutex_init(&bdev->bd_holder_lock);
-	bdev->bd_partno = partno;
+	atomic_set(&bdev->__bd_flags, partno);
 	bdev->bd_inode = inode;
 	bdev->bd_queue = disk->queue;
 	if (partno)

include/linux/blk_types.h

@@ -45,8 +45,9 @@ struct block_device {
 	struct request_queue *	bd_queue;
 	struct disk_stats __percpu *bd_stats;
 	unsigned long		bd_stamp;
+	atomic_t		__bd_flags;	// partition number + flags
+#define BD_PARTNO		255	// lower 8 bits; assign-once
 	bool			bd_read_only;	/* read-only policy */
-	u8			bd_partno;
 	bool			bd_write_holder;
 	bool			bd_has_submit_bio;
 	dev_t			bd_dev;

include/linux/blkdev.h

@@ -722,7 +722,22 @@ void disk_uevent(struct gendisk *disk, enum kobject_action action);
 
 static inline u8 bdev_partno(const struct block_device *bdev)
 {
-	return bdev->bd_partno;
+	return atomic_read(&bdev->__bd_flags) & BD_PARTNO;
+}
+
+static inline bool bdev_test_flag(const struct block_device *bdev, unsigned flag)
+{
+	return atomic_read(&bdev->__bd_flags) & flag;
+}
+
+static inline void bdev_set_flag(struct block_device *bdev, unsigned flag)
+{
+	atomic_or(flag, &bdev->__bd_flags);
+}
+
+static inline void bdev_clear_flag(struct block_device *bdev, unsigned flag)
+{
+	atomic_andnot(flag, &bdev->__bd_flags);
 }
 
 static inline int get_disk_ro(struct gendisk *disk)