On Mon, 09 Jun 2014 20:28:10 -0000
Thomas Gleixner <t...@linutronix.de> wrote:
> In case the dead lock detector is enabled we follow the lock chain to
> the end in rt_mutex_adjust_prio_chain, even if we could stop earlier
> due to the priority/waiter constellation.
>
> But once we are not longer the top priority waiter in a certain step
> or the task holding the lock has already the same priority then there
> is no point in dequeing and enqueing along the lock chain as there is
> no change at all.
>
> So stop the requeueing at this point.
>
> Signed-off-by: Thomas Gleixner <t...@linutronix.de>
> ---
> kernel/locking/rtmutex.c | 61
> +++++++++++++++++++++++++++++++++++++++++------
> 1 file changed, 54 insertions(+), 7 deletions(-)
>
> Index: tip/kernel/locking/rtmutex.c
> ===================================================================
> --- tip.orig/kernel/locking/rtmutex.c
> +++ tip/kernel/locking/rtmutex.c
> @@ -359,6 +359,7 @@ static int rt_mutex_adjust_prio_chain(st
> struct rt_mutex *lock;
> bool detect_deadlock;
> unsigned long flags;
> + bool requeue = true;
>
> detect_deadlock = rt_mutex_cond_detect_deadlock(orig_waiter, chwalk);
>
> @@ -436,18 +437,31 @@ static int rt_mutex_adjust_prio_chain(st
> goto out_unlock_pi;
> /*
> * If deadlock detection is off, we stop here if we
> - * are not the top pi waiter of the task.
> + * are not the top pi waiter of the task. If deadlock
> + * detection is enabled we continue, but stop the
> + * requeueing in the chain walk.
> */
> - if (!detect_deadlock && top_waiter != task_top_pi_waiter(task))
> - goto out_unlock_pi;
> + if (top_waiter != task_top_pi_waiter(task)) {
> + if (!detect_deadlock)
> + goto out_unlock_pi;
> + else
> + requeue = false;
> + }
> }
>
> /*
> - * When deadlock detection is off then we check, if further
> - * priority adjustment is necessary.
> + * If the waiter priority is the same as the task priority
> + * then there is no further priority adjustment necessary. If
> + * deadlock detection is off, we stop the chain walk. If its
> + * enabled we continue, but stop the requeueing in the chain
> + * walk.
> */
> - if (!detect_deadlock && waiter->prio == task->prio)
> - goto out_unlock_pi;
> + if (waiter->prio == task->prio) {
> + if (!detect_deadlock)
> + goto out_unlock_pi;
> + else
> + requeue = false;
> + }
>
> /*
> * We need to trylock here as we are holding task->pi_lock,
> @@ -475,6 +489,39 @@ static int rt_mutex_adjust_prio_chain(st
> }
>
> /*
> + * If we just follow the lock chain for deadlock detection, no
> + * need to do all the requeue operations. We avoid a truckload
s/We/To/
> + * of conditinals around the various places below and just do
s/ and/, /
> + * the minimum chain walk checks here.
> + */
> + if (!requeue) {
> + /* Release the task */
> + raw_spin_unlock_irqrestore(&task->pi_lock, flags);
> + put_task_struct(task);
> +
> + /* If there is no owner of the lock, end of chain. */
> + if (!rt_mutex_owner(lock)) {
> + raw_spin_unlock(&lock->wait_lock);
> + return 0;
> + }
> +
> + /* Grab the next task, i.e. owner of @lock */
> + task = rt_mutex_owner(lock);
> + get_task_struct(task);
> + raw_spin_lock_irqsave(&task->pi_lock, flags);
> +
> + /* Store whether owner is blocked itself and drop locks */
> + next_lock = task_blocked_on(task);
> + raw_spin_unlock_irqrestore(&task->pi_lock, flags);
> + raw_spin_unlock(&lock->wait_lock);
> +
> + /* If owner is not blocked, end of chain. */
> + if (!next_lock)
> + goto out_put_task;
On the loop back around, have something like:
if (top_waiter) {
if (!task_has_pi_waiters(task))
goto out_unlock_pi;
if (!requeue &&
top_waiter != task_top_pi_waiter(task)) {
if (!detect_deadlock)
goto out_unlock_pi;
else
requeue = false;
}
}
??
> + goto again;
> + }
> +
> + /*
> * Store the current top waiter before doing the requeue
> * operation on @lock. We need it for the boost/deboost
> * decision below.
>
--
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