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

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.

static void fixup_rt_mutex_waiters(struct rt_mutex *lock)
{
if (!rt_mutex_has_waiters(lock))
clear_rt_mutex_waiters(lock);
}
So it only clears the bit if there are no waiters. Yep, that should be
fine. The task clearing the bit is the owner and you have the wait_lock.
This should work.

-- Steve


> +
> + spin_unlock(&lock->wait_lock);
> +
> + /*
> + * Readjust priority, when we did not get the lock. We might have been
> + * the pending owner and boosted. Since we did not take the lock, the
> + * PI boost has to go.
> + */
> + if (unlikely(ret))
> + rt_mutex_adjust_prio(current);
> +
> + return ret;
> +}




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