Re: [RFC][PATCH 2/8] sched/rtmutex/deadline: Fix a PI crash for deadline tasks

[Steven Rostedt <rostedt@goodmis.org>]

On Tue, 07 Jun 2016 21:56:37 +0200
Peter Zijlstra <peterz@infradead.org> wrote:
\
> --- a/include/linux/sched/rt.h
> +++ b/include/linux/sched/rt.h
> @@ -19,6 +19,7 @@ static inline int rt_task(struct task_st
>  extern int rt_mutex_getprio(struct task_struct *p);
>  extern void rt_mutex_setprio(struct task_struct *p, int prio);
>  extern int rt_mutex_get_effective_prio(struct task_struct *task, int newprio);
> +extern void rt_mutex_update_top_task(struct task_struct *p);
>  extern struct task_struct *rt_mutex_get_top_task(struct task_struct *task);
>  extern void rt_mutex_adjust_pi(struct task_struct *p);
>  static inline bool tsk_is_pi_blocked(struct task_struct *tsk)
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1219,6 +1219,7 @@ static void rt_mutex_init_task(struct ta
>  #ifdef CONFIG_RT_MUTEXES
>  	p->pi_waiters = RB_ROOT;
>  	p->pi_waiters_leftmost = NULL;
> +	p->pi_top_task = NULL;
>  	p->pi_blocked_on = NULL;
>  #endif
>  }
> --- a/kernel/locking/rtmutex.c
> +++ b/kernel/locking/rtmutex.c
> @@ -256,6 +256,16 @@ rt_mutex_dequeue_pi(struct task_struct *
>  	RB_CLEAR_NODE(&waiter->pi_tree_entry);
>  }
>  
> +void rt_mutex_update_top_task(struct task_struct *p)
> +{
> +	if (!task_has_pi_waiters(p)) {
> +		p->pi_top_task = NULL;
> +		return;
> +	}
> +
> +	p->pi_top_task = task_top_pi_waiter(p)->task;
> +}
> +
>  /*
>   * Calculate task priority from the waiter tree priority
>   *
> @@ -273,10 +283,7 @@ int rt_mutex_getprio(struct task_struct
>  
>  struct task_struct *rt_mutex_get_top_task(struct task_struct *task)
>  {
> -	if (likely(!task_has_pi_waiters(task)))
> -		return NULL;
> -
> -	return task_top_pi_waiter(task)->task;
> +	return task->pi_top_task;
>  }
>  
>  /*
> @@ -285,12 +292,12 @@ struct task_struct *rt_mutex_get_top_tas
>   */
>  int rt_mutex_get_effective_prio(struct task_struct *task, int newprio)
>  {
> -	if (!task_has_pi_waiters(task))
> +	struct task_struct *top_task = rt_mutex_get_top_task(task);
> +
> +	if (!top_task)
>  		return newprio;
>  
> -	if (task_top_pi_waiter(task)->task->prio <= newprio)
> -		return task_top_pi_waiter(task)->task->prio;
> -	return newprio;
> +	return min(top_task->prio, newprio);
>  }
>  
>  /*
> @@ -307,24 +314,6 @@ static void __rt_mutex_adjust_prio(struc
>  }
>  
>  /*
> - * Adjust task priority (undo boosting). Called from the exit path of
> - * rt_mutex_slowunlock() and rt_mutex_slowlock().
> - *
> - * (Note: We do this outside of the protection of lock->wait_lock to
> - * allow the lock to be taken while or before we readjust the priority
> - * of task. We do not use the spin_xx_mutex() variants here as we are
> - * outside of the debug path.)
> - */
> -void rt_mutex_adjust_prio(struct task_struct *task)
> -{
> -	unsigned long flags;
> -
> -	raw_spin_lock_irqsave(&task->pi_lock, flags);
> -	__rt_mutex_adjust_prio(task);
> -	raw_spin_unlock_irqrestore(&task->pi_lock, flags);
> -}
> -
> -/*
>   * Deadlock detection is conditional:
>   *
>   * If CONFIG_DEBUG_RT_MUTEXES=n, deadlock detection is only conducted
> @@ -987,6 +976,7 @@ static void mark_wakeup_next_waiter(stru
>  	 * lock->wait_lock.
>  	 */
>  	rt_mutex_dequeue_pi(current, waiter);
> +	__rt_mutex_adjust_prio(current);
>  
>  	/*
>  	 * As we are waking up the top waiter, and the waiter stays
> @@ -1325,6 +1315,16 @@ static bool __sched rt_mutex_slowunlock(
>  	 */
>  	mark_wakeup_next_waiter(wake_q, lock);
>  
> +	/*
> +	 * We should deboost before waking the top waiter task such that
> +	 * we don't run two tasks with the 'same' priority. This however
> +	 * can lead to prio-inversion if we would get preempted after
> +	 * the deboost but before waking our high-prio task, hence the
> +	 * preempt_disable before unlock. Pairs with preempt_enable() in
> +	 * rt_mutex_postunlock();
> +	 */
> +	preempt_disable();
> +

This looks like a possible maintenance nightmare. Can we add some more
comments at the start of the functions that state that
rt_mutex_slowunlock() calls must be paired with rt_mutex_postunlock()?

Other than that...

Acked-by: Steven Rostedt <rostedt@goodmis.org>

-- Steve


>  	raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
>  
>  	/* check PI boosting */
> @@ -1400,20 +1400,9 @@ rt_mutex_fastunlock(struct rt_mutex *loc
>   */
>  void rt_mutex_postunlock(struct wake_q_head *wake_q, bool deboost)
>  {
> -	/*
> -	 * We should deboost before waking the top waiter task such that
> -	 * we don't run two tasks with the 'same' priority. This however
> -	 * can lead to prio-inversion if we would get preempted after
> -	 * the deboost but before waking our high-prio task, hence the
> -	 * preempt_disable.
> -	 */
> -	if (deboost) {
> -		preempt_disable();
> -		rt_mutex_adjust_prio(current);
> -	}
> -
>  	wake_up_q(wake_q);
>  
> +	/* Pairs with preempt_disable() in rt_mutex_slowunlock() */
>  	if (deboost)
>  		preempt_enable();
>  }
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3568,6 +3568,8 @@ void rt_mutex_setprio(struct task_struct
>  		goto out_unlock;
>  	}
>  
> +	rt_mutex_update_top_task(p);
> +
>  	trace_sched_pi_setprio(p, prio);
>  	oldprio = p->prio;
>  
>