[PATCH 4.4 38/84] futex: Fix futex lock the wrong page

[Greg Kroah-Hartman]

From: ZhangXiaoxu <zhangxiaoxu5@xxxxxxxxxx>

The upstram commit 65d8fc777f6d ("futex: Remove requirement
for lock_page() in get_futex_key()") use variable 'page' as
the page head, when merge it to stable branch, the variable
`page_head` is page head.

In the stable branch, the variable `page` not means the page
head, when lock the page head, we should lock 'page_head',
rather than 'page'.

It maybe lead a hung task problem.

Signed-off-by: ZhangXiaoxu <zhangxiaoxu5@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 kernel/futex.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -593,8 +593,8 @@ again:
 		 * applies. If this is really a shmem page then the page lock
 		 * will prevent unexpected transitions.
 		 */
-		lock_page(page);
-		shmem_swizzled = PageSwapCache(page) || page->mapping;
+		lock_page(page_head);
+		shmem_swizzled = PageSwapCache(page_head) || page_head->mapping;
 		unlock_page(page_head);
 		put_page(page_head);