block: add a disk_openers helper

Add a helper that returns the openers for a given gendisk to avoid having
drivers poke into disk->part0 to get at this information in a somewhat
cumbersome way.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220330052917.2566582-5-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/nbd.c

@@ -1219,7 +1219,7 @@ static int nbd_reconnect_socket(struct nbd_device *nbd, unsigned long arg)
 
 static void nbd_bdev_reset(struct nbd_device *nbd)
 {
-	if (nbd->disk->part0->bd_openers > 1)
+	if (disk_openers(nbd->disk) > 1)
 		return;
 	set_capacity(nbd->disk, 0);
 }
@@ -1576,7 +1576,7 @@ static void nbd_release(struct gendisk *disk, fmode_t mode)
 	struct nbd_device *nbd = disk->private_data;
 
 	if (test_bit(NBD_RT_DISCONNECT_ON_CLOSE, &nbd->config->runtime_flags) &&
-			disk->part0->bd_openers == 0)
+			disk_openers(disk) == 0)
 		nbd_disconnect_and_put(nbd);
 
 	nbd_config_put(nbd);

drivers/block/zram/zram_drv.c

@@ -1800,7 +1800,7 @@ static ssize_t reset_store(struct device *dev,
 
 	mutex_lock(&disk->open_mutex);
 	/* Do not reset an active device or claimed device */
-	if (disk->part0->bd_openers || zram->claim) {
+	if (disk_openers(disk) || zram->claim) {
 		mutex_unlock(&disk->open_mutex);
 		return -EBUSY;
 	}
@@ -1989,7 +1989,7 @@ static int zram_remove(struct zram *zram)
 	bool claimed;
 
 	mutex_lock(&zram->disk->open_mutex);
-	if (zram->disk->part0->bd_openers) {
+	if (disk_openers(zram->disk)) {
 		mutex_unlock(&zram->disk->open_mutex);
 		return -EBUSY;
 	}

include/linux/blkdev.h

@@ -176,6 +176,21 @@ static inline bool disk_live(struct gendisk *disk)
 	return !inode_unhashed(disk->part0->bd_inode);
 }
 
+/**
+ * disk_openers - returns how many openers are there for a disk
+ * @disk: disk to check
+ *
+ * This returns the number of openers for a disk.  Note that this value is only
+ * stable if disk->open_mutex is held.
+ *
+ * Note: Due to a quirk in the block layer open code, each open partition is
+ * only counted once even if there are multiple openers.
+ */
+static inline unsigned int disk_openers(struct gendisk *disk)
+{
+	return disk->part0->bd_openers;
+}
+
 /*
  * The gendisk is refcounted by the part0 block_device, and the bd_device
  * therein is also used for device model presentation in sysfs.