block: Fix oops in locked_inode_to_wb_and_lock_list()

When block device is closed, we call inode_detach_wb() in __blkdev_put()
which sets inode->i_wb to NULL. That is contrary to expectations that
inode->i_wb stays valid once set during the whole inode's lifetime and
leads to oops in wb_get() in locked_inode_to_wb_and_lock_list() because
inode_to_wb() returned NULL.

The reason why we called inode_detach_wb() is not valid anymore though.
BDI is guaranteed to stay along until we call bdi_put() from
bdev_evict_inode() so we can postpone calling inode_detach_wb() to that
moment.

Also add a warning to catch if someone uses inode_detach_wb() in a
dangerous way.

Reported-by: Thiago Jung Bauermann <bauerman@linux.vnet.ibm.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 2 insertions, 6 deletions

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 53e2389ae4d4..f2d59f143ef4 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -885,6 +885,8 @@ static void bdev_evict_inode(struct inode *inode)
         spin_lock(&bdev_lock);
         list_del_init(&bdev->bd_list);
         spin_unlock(&bdev_lock);
+        /* Detach inode from wb early as bdi_put() may free bdi->wb */
+        inode_detach_wb(inode);
         if (bdev->bd_bdi != &noop_backing_dev_info) {
                 bdi_put(bdev->bd_bdi);
                 bdev->bd_bdi = &noop_backing_dev_info;
@@ -1875,12 +1877,6 @@ static void __blkdev_put(struct block_device *bdev, fmode_t mode, int for_part)
                 kill_bdev(bdev);
 
                 bdev_write_inode(bdev);
-                /*
-                 * 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.
-                 */
-                inode_detach_wb(bdev->bd_inode);
         }
         if (bdev->bd_contains == bdev) {
                 if (disk->fops->release)