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    SubjectRe: [PATCH RFC] rtmutex: Permit rt_mutex_unlock() to be invoked with irqs disabled
    On Sun, Jul 24, 2011 at 11:00:41AM +0200, Thomas Gleixner wrote:
    > On Sat, 23 Jul 2011, Paul E. McKenney wrote:
    > > On Sun, Jul 24, 2011 at 02:05:13AM +0200, Thomas Gleixner wrote:
    > > > On Sun, 24 Jul 2011, Thomas Gleixner wrote:
    > > > > > > Thomas, I'm inclined to merge this, any objections?
    > > > > >
    > > > > > FWIW, it has been passing tests here.
    > > > >
    > > > > If it's only the unlock path, I'm fine with that change.
    > > > >
    > > > > Acked-by-me
    > > >
    > > > Hrmpft. That's requiring all places to take the lock irq safe. Not
    > > > really amused. For -RT that's a hotpath and we can really do without
    > > > the irq fiddling there. That needs a bit more thought.
    > >
    > > Indeed... If I make only some of the lock acquisitions irq safe, lockdep
    > > will yell at me. And rightfully so, as that could result in deadlock.
    > >
    > > So, what did you have in mind?
    >
    > Have no real good idea yet for this. Could you grab rt and check
    > whether you can observe any impact when the patch is applied?

    Hmmm, wait a minute... There might be a way to do this with zero
    impact on the fastpath, given that I am allocating an rt_mutex on
    the stack that is used only by RCU priority boosting, and that only
    rt_mutex_init_proxy_locked(), rt_mutex_lock(), and rt_mutex_unlock()
    are used.

    So I could do the following:

    o Use lockdep_set_class_and_name() to make the ->wait_lock()
    field of my rt_mutex have a separate lockdep class. I guess
    I should allocate a global variable for lock_class_key
    rather than allocating it on the stack. ;-)

    o Make all calls from RCU priority boosting to rt_mutex_lock()
    and rt_mutex_unlock() have irqs disabled.

    o Make __rt_mutex_slowlock() do the following when sleeping:

    raw_spin_unlock(&lock->wait_lock);

    debug_rt_mutex_print_deadlock(waiter);

    {
    int was_disabled = irqs_disabled();

    if (was_disabled)
    local_irq_enable();

    schedule_rt_mutex(lock);

    if (was_disabled)
    local_irq_disable();

    }

    raw_spin_lock(&lock->wait_lock);
    set_current_state(state);

    Would that work reasonably?

    Thanx, Paul


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