On Tue 21-02-17 19:53:29, Bart Van Assche wrote:

On 02/21/2017 09:55 AM, Jan Kara wrote:

quoted

diff --git a/block/blk-core.c b/block/blk-core.c
index 47104f6a398b..9a901dcfdd5c 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c

@@ -580,8 +580,6 @@ void blk_cleanup_queue(struct request_queue *q)
 		q->queue_lock = &q->__queue_lock;
 	spin_unlock_irq(lock);
 
-	bdi_unregister(q->backing_dev_info);
-
 	/* @q is and will stay empty, shutdown and put */
 	blk_put_queue(q);
 }

diff --git a/block/genhd.c b/block/genhd.c
index f6c4d4400759..68c613edb93a 100644
--- a/block/genhd.c
+++ b/block/genhd.c

@@ -660,6 +660,13 @@ void del_gendisk(struct gendisk *disk)
 	disk->flags &= ~GENHD_FL_UP;
 
 	sysfs_remove_link(&disk_to_dev(disk)->kobj, "bdi");
+	/*
+	 * Unregister bdi before releasing device numbers (as they can get
+	 * reused and we'd get clashes in sysfs) but after bdev inodes are
+	 * unhashed and thus will be soon destroyed as bdev inode's reference
+	 * to wb_writeback can block bdi_unregister().
+	 */
+	bdi_unregister(disk->queue->backing_dev_info);
 	blk_unregister_queue(disk);
 	blk_unregister_region(disk_devt(disk), disk->minors);

This change looks suspicious to me. There are drivers that create a
block layer queue but neither call device_add_disk() nor del_gendisk(),
e.g. drivers/scsi/st.c. Although bdi_init() will be called for the
queues created by these drivers, this patch will cause the
bdi_unregister() call to be skipped for these drivers.

Well, the thing is that bdi_unregister() is the counterpart to
bdi_register(). Unless you call bdi_register(), which happens only in
device_add_disk() (and some filesystems which create their private bdis),
there's no point in calling bdi_unregister(). Counterpart to bdi_init() is
bdi_exit() and that gets called always once bdi reference count drops to 0.

								Honza
-- 
Jan Kara [off-list ref]
SUSE Labs, CR