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    Subject[PATCH 4.4 38/84] futex: Fix futex lock the wrong page
    Date
    From: ZhangXiaoxu <zhangxiaoxu5@huawei.com>

    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@huawei.com>
    Cc: stable@vger.kernel.org
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    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);


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