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Date: Tue, 14 Jun 2016 11:21:09 +0100
From: Juri Lelli <juri.lelli@....com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: mingo@...nel.org, tglx@...utronix.de, rostedt@...dmis.org,
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
Hi,
On 07/06/16 21:56, Peter Zijlstra wrote:
> From: Xunlei Pang <xlpang@...hat.com>
>
> A crash happened while I was playing with deadline PI rtmutex.
>
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
> IP: [<ffffffff810eeb8f>] rt_mutex_get_top_task+0x1f/0x30
> PGD 232a75067 PUD 230947067 PMD 0
> Oops: 0000 [#1] SMP
> CPU: 1 PID: 10994 Comm: a.out Not tainted
>
> Call Trace:
> [<ffffffff810b658c>] enqueue_task+0x2c/0x80
> [<ffffffff810ba763>] activate_task+0x23/0x30
> [<ffffffff810d0ab5>] pull_dl_task+0x1d5/0x260
> [<ffffffff810d0be6>] pre_schedule_dl+0x16/0x20
> [<ffffffff8164e783>] __schedule+0xd3/0x900
> [<ffffffff8164efd9>] schedule+0x29/0x70
> [<ffffffff8165035b>] __rt_mutex_slowlock+0x4b/0xc0
> [<ffffffff81650501>] rt_mutex_slowlock+0xd1/0x190
> [<ffffffff810eeb33>] rt_mutex_timed_lock+0x53/0x60
> [<ffffffff810ecbfc>] futex_lock_pi.isra.18+0x28c/0x390
> [<ffffffff810ed8b0>] do_futex+0x190/0x5b0
> [<ffffffff810edd50>] SyS_futex+0x80/0x180
>
This seems to be caused by the race condition you detail below between
load balancing and PI code. I tried to reproduce the BUG on my box, but
it looks hard to get. Do you have a reproducer I can give a try?
> This is because rt_mutex_enqueue_pi() and rt_mutex_dequeue_pi()
> are only protected by pi_lock when operating pi waiters, while
> rt_mutex_get_top_task(), will access them with rq lock held but
> not holding pi_lock.
>
> In order to tackle it, we introduce new "pi_top_task" pointer
> cached in task_struct, and add new rt_mutex_update_top_task()
> to update its value, it can be called by rt_mutex_setprio()
> which held both owner's pi_lock and rq lock. Thus "pi_top_task"
> can be safely accessed by enqueue_task_dl() under rq lock.
>
> [XXX this next section is unparsable]
Yes, a bit hard to understand. However, am I correct in assuming this
patch and the previous one should fix this problem? Or are there still
other races causing issues?
> One problem is when rt_mutex_adjust_prio()->...->rt_mutex_setprio(),
> at that time rtmutex lock was released and owner was marked off,
> this can cause "pi_top_task" dereferenced to be a running one(as it
> can be falsely woken up by others before rt_mutex_setprio() is
> made to update "pi_top_task"). We solve this by directly calling
> __rt_mutex_adjust_prio() in mark_wakeup_next_waiter() which held
> pi_lock and rtmutex lock, and remove rt_mutex_adjust_prio(). Since
> now we moved the deboost point, in order to avoid current to be
> preempted due to deboost earlier before wake_up_q(), we also moved
> preempt_disable() before unlocking rtmutex.
>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Juri Lelli <juri.lelli@....com>
> Originally-From: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Xunlei Pang <xlpang@...hat.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Link: http://lkml.kernel.org/r/1461659449-19497-2-git-send-email-xlpang@redhat.com
The idea of this fix makes sense to me. But, I would like to be able to
see the BUG and test the fix. What I have is a test in which I create N
DEADLINE workers that share a PI mutex. They get migrated around and
seem to stress PI code. But I couldn't hit the BUG yet. Maybe I let it
run for some more time.
Best,
- Juri
> ---
>
> include/linux/init_task.h | 1
> include/linux/sched.h | 2 +
> include/linux/sched/rt.h | 1
> kernel/fork.c | 1
> kernel/locking/rtmutex.c | 65 +++++++++++++++++++---------------------------
> kernel/sched/core.c | 2 +
> 6 files changed, 34 insertions(+), 38 deletions(-)
>
> --- a/include/linux/init_task.h
> +++ b/include/linux/init_task.h
> @@ -162,6 +162,7 @@ extern struct task_group root_task_group
> #ifdef CONFIG_RT_MUTEXES
> # define INIT_RT_MUTEXES(tsk) \
> .pi_waiters = RB_ROOT, \
> + .pi_top_task = NULL, \
> .pi_waiters_leftmost = NULL,
> #else
> # define INIT_RT_MUTEXES(tsk)
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1681,6 +1681,8 @@ struct task_struct {
> /* PI waiters blocked on a rt_mutex held by this task */
> struct rb_root pi_waiters;
> struct rb_node *pi_waiters_leftmost;
> + /* Updated under owner's pi_lock and rq lock */
> + struct task_struct *pi_top_task;
> /* Deadlock detection and priority inheritance handling */
> struct rt_mutex_waiter *pi_blocked_on;
> #endif
> --- 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();
> +
> 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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