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    Subject[12/99] futex: Fix spurious wakeup for requeue_pi really
    2.6.31-stable review patch.  If anyone has any objections, please let us know.

    ------------------
    From: Thomas Gleixner <tglx@linutronix.de>

    commit 11df6dddcbc38affb7473aad3d962baf8414a947 upstream.

    The requeue_pi path doesn't use unqueue_me() (and the racy lock_ptr ==
    NULL test) nor does it use the wake_list of futex_wake() which where
    the reason for commit 41890f2 (futex: Handle spurious wake up)

    See debugging discussing on LKML Message-ID: <4AD4080C.20703@us.ibm.com>

    The changes in this fix to the wait_requeue_pi path were considered to
    be a likely unecessary, but harmless safety net. But it turns out that
    due to the fact that for unknown $@#!*( reasons EWOULDBLOCK is defined
    as EAGAIN we built an endless loop in the code path which returns
    correctly EWOULDBLOCK.

    Spurious wakeups in wait_requeue_pi code path are unlikely so we do
    the easy solution and return EWOULDBLOCK^WEAGAIN to user space and let
    it deal with the spurious wakeup.

    Cc: Darren Hart <dvhltc@us.ibm.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Eric Dumazet <eric.dumazet@gmail.com>
    Cc: John Stultz <johnstul@linux.vnet.ibm.com>
    Cc: Dinakar Guniguntala <dino@in.ibm.com>
    LKML-Reference: <4AE23C74.1090502@us.ibm.com>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

    ---
    kernel/futex.c | 6 +-----
    1 file changed, 1 insertion(+), 5 deletions(-)

    --- a/kernel/futex.c
    +++ b/kernel/futex.c
    @@ -2103,7 +2103,7 @@ int handle_early_requeue_pi_wakeup(struc
    plist_del(&q->list, &q->list.plist);

    /* Handle spurious wakeups gracefully */
    - ret = -EAGAIN;
    + ret = -EWOULDBLOCK;
    if (timeout && !timeout->task)
    ret = -ETIMEDOUT;
    else if (signal_pending(current))
    @@ -2184,7 +2184,6 @@ static int futex_wait_requeue_pi(u32 __u
    debug_rt_mutex_init_waiter(&rt_waiter);
    rt_waiter.task = NULL;

    -retry:
    key2 = FUTEX_KEY_INIT;
    ret = get_futex_key(uaddr2, fshared, &key2, VERIFY_WRITE);
    if (unlikely(ret != 0))
    @@ -2282,9 +2281,6 @@ out_put_keys:
    out_key2:
    put_futex_key(fshared, &key2);

    - /* Spurious wakeup ? */
    - if (ret == -EAGAIN)
    - goto retry;
    out:
    if (to) {
    hrtimer_cancel(&to->timer);



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