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    SubjectRe: [PATCH 4/4] futex: convert hash_bucket locks to raw_spinlock_t
    On Sat, 2010-07-10 at 21:41 +0200, Mike Galbraith wrote:
    > On Fri, 2010-07-09 at 15:33 -0700, Darren Hart wrote:

    > > If we can't move the unlock above before set_owner, then we may need a:
    > >
    > > retry:
    > > cur->lock()
    > > top_waiter = get_top_waiter()
    > > cur->unlock()
    > >
    > > double_lock(cur, topwaiter)
    > > if top_waiter != get_top_waiter()
    > > double_unlock(cur, topwaiter)
    > > goto retry
    > >
    > > Not ideal, but I think I prefer that to making all the hb locks raw.

    Another option: only scratch the itchy spot.

    futex: non-blocking synchronization point for futex_wait_requeue_pi() and futex_requeue().

    Problem analysis by Darren Hart;
    The requeue_pi mechanism introduced proxy locking of the rtmutex. This creates
    a scenario where a task can wake-up, not knowing it has been enqueued on an
    rtmutex. In order to detect this, the task would have to be able to take either
    task->pi_blocked_on->lock->wait_lock and/or the hb->lock. Unfortunately,
    without already holding one of these, the pi_blocked_on variable can change
    from NULL to valid or from valid to NULL. Therefor, the task cannot be allowed
    to take a sleeping lock after wakeup or it could end up trying to block on two
    locks, the second overwriting a valid pi_blocked_on value. This obviously
    breaks the pi mechanism.

    Rather than convert the bh-lock to a raw spinlock, do so only in the spot where
    blocking cannot be allowed, ie before we know that lock handoff has completed.

    Signed-off-by: Mike Galbraith <>
    Cc: Darren Hart <>
    Cc: Thomas Gleixner <>
    Cc: Peter Zijlstra <>
    Cc: Ingo Molnar <>
    Cc: Eric Dumazet <>
    Cc: John Kacur <>
    Cc: Steven Rostedt <>

    diff --git a/kernel/futex.c b/kernel/futex.c
    index a6cec32..ef489f3 100644
    --- a/kernel/futex.c
    +++ b/kernel/futex.c
    @@ -2255,7 +2255,14 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, int fshared,
    /* Queue the futex_q, drop the hb lock, wait for wakeup. */
    futex_wait_queue_me(hb, &q, to);

    - spin_lock(&hb->lock);
    + /*
    + * Non-blocking synchronization point with futex_requeue().
    + *
    + * We dare not block here because this will alter PI state, possibly
    + * before our waker finishes modifying same in wakeup_next_waiter().
    + */
    + while(!spin_trylock(&hb->lock))
    + cpu_relax();
    ret = handle_early_requeue_pi_wakeup(hb, &q, &key2, to);
    if (ret)

     \ /
      Last update: 2010-07-11 15:35    [W:0.030 / U:11.232 seconds]
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