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

[Herton R. Krzesinski]

From: Tejun Heo <tj at kernel.org>

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 at kernel.org>
Reported-by: David Miller <davem at davemloft.net>
Tested-by: David Miller <davem at davemloft.net>
Cc: stable at kernel.org
Signed-off-by: Jens Axboe <jaxboe at fusionio.com>
(cherry picked from commit 4c49ff3fe128ca68dabd07537415c419ad7f82f9)
BugLink: http://bugs.launchpad.net/bugs/857170
Signed-off-by: Herton Ronaldo Krzesinski <herton.krzesinski at canonical.com>
---
 fs/block_dev.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 59277ba..0ce65c9 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -1267,8 +1267,8 @@ 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);
 		}
-- 
1.7.4.1