[PATCH 4.2.y-ckt 148/305] block: detach bdev inode from its wb in __blkdev_put()

From: Kamal Mostafa
Date: Fri Jan 15 2016 - 19:05:20 EST

4.2.8-ckt2 -stable review patch. If anyone has any objections, please let me know.


From: Ilya Dryomov <idryomov@xxxxxxxxx>

commit 43d1c0eb7e11919f85200d2fce211173526f7304 upstream.

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;
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;
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.

Signed-off-by: Ilya Dryomov <idryomov@xxxxxxxxx>
Acked-by: Tejun Heo <tj@xxxxxxxxxx>
Tested-by: Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxx>
[ kamal: backport to 4.2-stable: bdev_write_inode() takes an inode ]
Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
fs/block_dev.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 1170f8c..426b30f 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -1499,11 +1499,14 @@ static void __blkdev_put(struct block_device *bdev, fmode_t mode, int for_part)
+ bdev_write_inode(bdev->bd_inode);
- * ->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->bd_inode);
+ inode_detach_wb(bdev->bd_inode);
if (bdev->bd_contains == bdev) {
if (disk->fops->release)