Re: [TIP][RFC 6/7] futex: add requeue_pi calls

[Darren Hart]

Steven Rostedt wrote:
> On Thu, 5 Mar 2009, Darren Hart wrote:
> > As it turns out I missed setting RT_MUTEX_HAS_WAITERS on the rt_mutex in
> > rt_mutex_start_proxy_lock() - seems awfully silly in retrospect - but a
> > little non-obvious while writing it.  I added mark_rt_mutex_waiters()
> > after the call to task_blocks_on_rt_mutex() and the test has completed
> > more than 400 iterations successfully (it would fail after no more than
> > 2 most of the time before).
> >
> > Steven, there are several ways to set RT_MUTEX_HAS_WAITERS - but this
> > seemed like a reasonable approach, would you agree?  Since I'm holding
> > the wait_lock I don't technically need the atomic cmpxchg and could
> > probably just set it explicity - do you have a preference?
>
> > +
> > +/**
> > + * rt_mutex_finish_proxy_lock - Complete the taking of the lock initialized
> > on
> > + *                              our behalf by another thread.
> > + * @lock: the rt_mutex we were woken on
> > + * @to: the timeout, null if none. hrtimer should already have been started.
> > + * @waiter: the pre-initialized rt_mutex_waiter
> > + * @detect_deadlock: for use by __rt_mutex_slowlock
> > + *
> > + * Special API call for PI-futex requeue support
> > + */
> > +int rt_mutex_finish_proxy_lock(struct rt_mutex *lock,
> > +			       struct hrtimer_sleeper *to,
> > +			       struct rt_mutex_waiter *waiter,
> > +			       int detect_deadlock)
> > +{
> > +	int ret;
> > +
> > +	if (waiter->task)
> > +		schedule_rt_mutex(lock);
> > +
> > +	spin_lock(&lock->wait_lock);
> > +
> > +	set_current_state(TASK_INTERRUPTIBLE);
> > +
> > +	ret = __rt_mutex_slowlock(lock, TASK_INTERRUPTIBLE, to, waiter,
> > +				  detect_deadlock);
> > +
> > +	set_current_state(TASK_RUNNING);
> > +
> > +	if (unlikely(waiter->task))
> > +		remove_waiter(lock, waiter);
> > +
> > +	/*
> > +	 * try_to_take_rt_mutex() sets the waiter bit unconditionally. We
> > might
> > +	 * have to fix that up.
> > +	 */
> > +	fixup_rt_mutex_waiters(lock);
>
> Darren,
>
> I take it you are talking about the above.

Actually no, I was talking about rt_mutex_START_proxy_lock():

/**
* rt_mutex_start_proxy_lock - prepare another task to take the lock
*
* @lock:		the rt_mutex to take
* @waiter:		the rt_mutex_waiter initialized by the waiter
* @task:		the task to prepare
* @detext_deadlock:	passed to task_blocks_on_rt_mutex
*
* The lock should have an owner, and it should not be task.
* Special API call for FUTEX_REQUEUE_PI support.
*/
int rt_mutex_start_proxy_lock(struct rt_mutex *lock,
			      struct rt_mutex_waiter *waiter,
			      struct task_struct *task, int detect_deadlock)
{
	int ret;

	spin_lock(&lock->wait_lock);
	ret = task_blocks_on_rt_mutex(lock, waiter, task, detect_deadlock);


I add the following line to fix the bug.  Question is, should I use this atomic
optimization here (under the lock->wait_lock) or should I just do 
"lock->owner |= RT_MUTEX_HAS_WAITERS" ?

=====>	mark_rt_mutex_waiters(lock);

	if (ret && !waiter->task) {
		/*
		 * Reset the return value. We might have
		 * returned with -EDEADLK and the owner
		 * released the lock while we were walking the
		 * pi chain.  Let the waiter sort it out.
		 */
		ret = 0;
	}
	spin_unlock(&lock->wait_lock);

	debug_rt_mutex_print_deadlock(waiter);

	return ret;
}



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