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Date: Tue, 14 Jun 2016 14:42:17 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: mingo@...nel.org, tglx@...utronix.de, juri.lelli@....com,
xlpang@...hat.com, linux-kernel@...r.kernel.org,
mathieu.desnoyers@...icios.com, jdesfossez@...icios.com,
bristot@...hat.com, Ingo Molnar <mingo@...hat.com>
Subject: Re: [RFC][PATCH 2/8] sched/rtmutex/deadline: Fix a PI crash for
deadline tasks
On Tue, 07 Jun 2016 21:56:37 +0200
Peter Zijlstra <peterz@...radead.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@...dmis.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;
>
>
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