block: blkdev_get() should access ->bd_disk only after success

d4dc210f69 (block: don't block events on excl write for non-optical
devices) added dereferencing of bdev->bd_disk to test
GENHD_FL_BLOCK_EVENTS_ON_EXCL_WRITE; however, bdev->bd_disk can be
%NULL if open failed which can lead to an oops.

Test the flag after testing open was successful, not before.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: David Miller <davem@davemloft.net>
Tested-by: David Miller <davem@davemloft.net>
Cc: stable@kernel.org
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 1f2b19978333..1a2421f908f0 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -1272,8 +1272,8 @@ int blkdev_get(struct block_device *bdev, fmode_t mode, void *holder)
                  * individual writeable reference is too fragile given the
                  * way @mode is used in blkdev_get/put().
                  */
-                if ((disk->flags & GENHD_FL_BLOCK_EVENTS_ON_EXCL_WRITE) &&
-                    !res && (mode & FMODE_WRITE) && !bdev->bd_write_holder) {
+                if (!res && (mode & FMODE_WRITE) && !bdev->bd_write_holder &&
+                    (disk->flags & GENHD_FL_BLOCK_EVENTS_ON_EXCL_WRITE)) {
                         bdev->bd_write_holder = true;
                         disk_block_events(disk);
                 }