block: turn bdev->bd_openers into an atomic_t

All manipulation of bd_openers is under disk->open_mutex and will remain
so for the foreseeable future.  But at least one place reads it without
the lock (blkdev_get) and there are more to be added.  So make sure the
compiler does not do turn the increments and decrements into non-atomic
sequences by using an atomic_t.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220330052917.2566582-6-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/bdev.c

@@ -673,17 +673,17 @@ static int blkdev_get_whole(struct block_device *bdev, fmode_t mode)
 		}
 	}
 
-	if (!bdev->bd_openers)
+	if (!atomic_read(&bdev->bd_openers))
 		set_init_blocksize(bdev);
 	if (test_bit(GD_NEED_PART_SCAN, &disk->state))
 		bdev_disk_changed(disk, false);
-	bdev->bd_openers++;
+	atomic_inc(&bdev->bd_openers);
 	return 0;
 }
 
 static void blkdev_put_whole(struct block_device *bdev, fmode_t mode)
 {
-	if (!--bdev->bd_openers)
+	if (atomic_dec_and_test(&bdev->bd_openers))
 		blkdev_flush_mapping(bdev);
 	if (bdev->bd_disk->fops->release)
 		bdev->bd_disk->fops->release(bdev->bd_disk, mode);
@@ -694,7 +694,7 @@ static int blkdev_get_part(struct block_device *part, fmode_t mode)
 	struct gendisk *disk = part->bd_disk;
 	int ret;
 
-	if (part->bd_openers)
+	if (atomic_read(&part->bd_openers))
 		goto done;
 
 	ret = blkdev_get_whole(bdev_whole(part), mode);
@@ -708,7 +708,7 @@ static int blkdev_get_part(struct block_device *part, fmode_t mode)
 	disk->open_partitions++;
 	set_init_blocksize(part);
 done:
-	part->bd_openers++;
+	atomic_inc(&part->bd_openers);
 	return 0;
 
 out_blkdev_put:
@@ -720,7 +720,7 @@ static void blkdev_put_part(struct block_device *part, fmode_t mode)
 {
 	struct block_device *whole = bdev_whole(part);
 
-	if (--part->bd_openers)
+	if (!atomic_dec_and_test(&part->bd_openers))
 		return;
 	blkdev_flush_mapping(part);
 	whole->bd_disk->open_partitions--;
@@ -899,7 +899,7 @@ void blkdev_put(struct block_device *bdev, fmode_t mode)
 	 * of the world and we want to avoid long (could be several minute)
 	 * syncs while holding the mutex.
 	 */
-	if (bdev->bd_openers == 1)
+	if (atomic_read(&bdev->bd_openers) == 1)
 		sync_blockdev(bdev);
 
 	mutex_lock(&disk->open_mutex);
@@ -1044,7 +1044,7 @@ void sync_bdevs(bool wait)
 		bdev = I_BDEV(inode);
 
 		mutex_lock(&bdev->bd_disk->open_mutex);
-		if (!bdev->bd_openers) {
+		if (!atomic_read(&bdev->bd_openers)) {
 			; /* skip */
 		} else if (wait) {
 			/*

block/partitions/core.c

@@ -478,7 +478,7 @@ int bdev_del_partition(struct gendisk *disk, int partno)
 		goto out_unlock;
 
 	ret = -EBUSY;
-	if (part->bd_openers)
+	if (atomic_read(&part->bd_openers))
 		goto out_unlock;
 
 	delete_partition(part);

include/linux/blk_types.h

@@ -44,7 +44,7 @@ struct block_device {
 	unsigned long		bd_stamp;
 	bool			bd_read_only;	/* read-only policy */
 	dev_t			bd_dev;
-	int			bd_openers;
+	atomic_t		bd_openers;
 	struct inode *		bd_inode;	/* will die */
 	struct super_block *	bd_super;
 	void *			bd_claiming;

include/linux/blkdev.h

@@ -188,7 +188,7 @@ static inline bool disk_live(struct gendisk *disk)
  */
 static inline unsigned int disk_openers(struct gendisk *disk)
 {
-	return disk->part0->bd_openers;
+	return atomic_read(&disk->part0->bd_openers);
 }
 
 /*