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    SubjectRe: [PATCH 4/4] futex: convert hash_bucket locks to raw_spinlock_t
    On 07/11/2010 06:33 AM, Mike Galbraith wrote:
    > 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.

    I like it. I especially like the change is only evident if you are using
    the code path that introduced the problem in the first place. If you're
    doing a lot of requeue_pi operations, then the waking waiters have an
    advantage over new pending waiters or other tasks with futex keyed on
    the same hash-bucket... but that seems acceptable to me.

    I'd like to confirm that holding the pendowner->pi-lock across the
    wakeup in wakeup_next_waiter() isn't feasible first. If it can work, I
    think the impact would be lower. I'll have a look tomorrow.

    Nice work Mike.

    --
    Darrem

    > Signed-off-by: Mike Galbraith<efault@gmx.de>
    > Cc: Darren Hart<dvhltc@us.ibm.com>
    > Cc: Thomas Gleixner<tglx@linutronix.de>
    > Cc: Peter Zijlstra<peterz@infradead.org>
    > Cc: Ingo Molnar<mingo@elte.hu>
    > Cc: Eric Dumazet<eric.dumazet@gmail.com>
    > Cc: John Kacur<jkacur@redhat.com>
    > Cc: Steven Rostedt<rostedt@goodmis.org>
    >
    > 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);
    > spin_unlock(&hb->lock);
    > if (ret)
    >
    >


    --
    Darren Hart
    IBM Linux Technology Center
    Real-Time Linux Team


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