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    Subject[PATCH 4.4 31/87] futex: Replace barrier() in unqueue_me() with READ_ONCE()
    Date
    4.4-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Jianyu Zhan <nasa4836@gmail.com>

    commit 29b75eb2d56a714190a93d7be4525e617591077a upstream.

    Commit e91467ecd1ef ("bug in futex unqueue_me") introduced a barrier() in
    unqueue_me() to prevent the compiler from rereading the lock pointer which
    might change after a check for NULL.

    Replace the barrier() with a READ_ONCE() for the following reasons:

    1) READ_ONCE() is a weaker form of barrier() that affects only the specific
    load operation, while barrier() is a general compiler level memory barrier.
    READ_ONCE() was not available at the time when the barrier was added.

    2) Aside of that READ_ONCE() is descriptive and self explainatory while a
    barrier without comment is not clear to the casual reader.

    No functional change.

    [ tglx: Massaged changelog ]

    Signed-off-by: Jianyu Zhan <nasa4836@gmail.com>
    Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
    Acked-by: Darren Hart <dvhart@linux.intel.com>
    Cc: dave@stgolabs.net
    Cc: peterz@infradead.org
    Cc: linux@rasmusvillemoes.dk
    Cc: akpm@linux-foundation.org
    Cc: fengguang.wu@intel.com
    Cc: bigeasy@linutronix.de
    Link: http://lkml.kernel.org/r/1457314344-5685-1-git-send-email-nasa4836@gmail.com
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    kernel/futex.c | 8 ++++++--
    1 file changed, 6 insertions(+), 2 deletions(-)

    --- a/kernel/futex.c
    +++ b/kernel/futex.c
    @@ -1939,8 +1939,12 @@ static int unqueue_me(struct futex_q *q)

    /* In the common case we don't take the spinlock, which is nice. */
    retry:
    - lock_ptr = q->lock_ptr;
    - barrier();
    + /*
    + * q->lock_ptr can change between this read and the following spin_lock.
    + * Use READ_ONCE to forbid the compiler from reloading q->lock_ptr and
    + * optimizing lock_ptr out of the logic below.
    + */
    + lock_ptr = READ_ONCE(q->lock_ptr);
    if (lock_ptr != NULL) {
    spin_lock(lock_ptr);
    /*

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