block: add a ->free_disk method

Add a method to notify the driver that the gendisk is about to be freed.
This allows drivers to tie the lifetime of their private data to that of
the gendisk and thus deal with device removal races without expensive
synchronization and boilerplate code.

A new flag is added so that ->free_disk is only called after a successful
call to add_disk, which significantly simplifies the error handling path
during probing.

Signed-off-by: Christoph Hellwig <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

block/genhd.c

···
 
 	disk_update_readahead(disk);
 	disk_add_events(disk);
+	set_bit(GD_ADDED, &disk->state);
 	return 0;
 
 out_unregister_bdi:
···
 	xa_destroy(&disk->part_tbl);
 	disk->queue->disk = NULL;
 	blk_put_queue(disk->queue);
+
+	if (test_bit(GD_ADDED, &disk->state) && disk->fops->free_disk)
+		disk->fops->free_disk(disk);
+
 	iput(disk->part0->bd_inode);	/* frees the disk */
 }
 

include/linux/blkdev.h

···
 #define GD_READ_ONLY			1
 #define GD_DEAD				2
 #define GD_NATIVE_CAPACITY		3
+#define GD_ADDED			4
 
 	struct mutex open_mutex;	/* open/close mutex */
 	unsigned open_partitions;	/* number of open partitions */
···
 	void (*unlock_native_capacity) (struct gendisk *);
 	int (*getgeo)(struct block_device *, struct hd_geometry *);
 	int (*set_read_only)(struct block_device *bdev, bool ro);
+	void (*free_disk)(struct gendisk *disk);
 	/* this callback is with swap_lock and sometimes page table lock held */
 	void (*swap_slot_free_notify) (struct block_device *, unsigned long);
 	int (*report_zones)(struct gendisk *, sector_t sector,