Messages in this thread | | | Date | Sun, 24 Jul 2011 08:56:34 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC] rtmutex: Permit rt_mutex_unlock() to be invoked with irqs disabled |
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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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