FAILED: patch "[PATCH] block: detach bdev inode from its wb in __blkdev_put()" failed to apply to 4.3-stable tree

The patch below does not apply to the 4.3-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From: Ilya Dryomov <[email protected]>
Date: Fri, 20 Nov 2015 22:22:34 +0100
Subject: [PATCH] block: detach bdev inode from its wb in __blkdev_put()

Since 52ebea749aae ("writeback: make backing_dev_info host cgroup-specific bdi_writebacks") inode, at some point in its lifetime, gets attached to a wb (struct bdi_writeback). Detaching happens on evict, in inode_detach_wb() called from __destroy_inode(), and involves updating wb.

However, detaching an internal bdev inode from its wb in __destroy_inode() is too late. Its bdi and by extension root wb are embedded into struct request_queue, which has different lifetime rules and can be freed long before the final bdput() is called (can be from __fput() of a corresponding /dev inode, through dput() - evict() - bd_forget(). bdevs hold onto the underlying disk/queue pair only while opened; as soon as bdev is closed all bets are off. In fact, disk/queue can be gone before __blkdev_put() even returns:

1499 static void __blkdev_put(struct block_device *bdev, fmode_t mode, int for_part) 1500 {
...
1518 if (bdev->bd_contains == bdev) {
1519 if (disk->fops->release)
1520 disk->fops->release(disk, mode);

[ Driver puts its references to disk/queue ]

1521 }
1522 if (!bdev->bd_openers) {
1523 struct module *owner = disk->fops->owner;
1524

1525                 disk_put_part(bdev->bd_part);
1526                 bdev->bd_part = NULL;
1527                 bdev->bd_disk = NULL;
1528                 if (bdev != bdev->bd_contains)

1529 victim = bdev->bd_contains;
1530 bdev->bd_contains = NULL;
1531
1532 put_disk(disk);

[ We put ours, the queue is gone
The last bdput() would result in a write to invalid memory ]

1533 module_put(owner);
...
1539 }

Since bdev inodes are special anyway, detach them in __blkdev_put() after clearing inode's dirty bits, turning the problematic inode_detach_wb() in __destroy_inode() into a noop.

add_disk() grabs its disk->queue since 523e1d399ce0 ("block: make gendisk hold a reference to its queue"), so the old ->release comment is removed in favor of the new inode_detach_wb() comment.

index c25639e907bd..44d4a1e9244e 100644
@@ -1523,11 +1523,14 @@ static void __blkdev_put(struct block_device *bdev, fmode_t mode, int for_part)
 		WARN_ON_ONCE(bdev->bd_holders);
 		sync_blockdev(bdev);
 		kill_bdev(bdev);
+
+		bdev_write_inode(bdev);
 		/*
-		 * ->release can cause the queue to disappear, so flush all
-		 * dirty data before.
+		 * Detaching bdev inode from its wb in __destroy_inode()
+		 * is too late: the queue which embeds its bdi (along with
+		 * root wb) can be gone as soon as we put_disk() below.
 		 */
-		bdev_write_inode(bdev);
+		inode_detach_wb(bdev->bd_inode);
 	}
 	if (bdev->bd_contains == bdev) {
 		if (disk->fops->release)