Re: NULL pointer dereference in pick_next_task_fair

[Kirill Tkhai <ktkhai@virtuozzo.com>]

On 07.11.2019 16:26, Peter Zijlstra wrote:
> On Thu, Nov 07, 2019 at 11:36:50AM +0300, Kirill Tkhai wrote:
>> On 06.11.2019 20:27, Peter Zijlstra wrote:
>>> On Wed, Nov 06, 2019 at 05:54:37PM +0100, Peter Zijlstra wrote:
>>>> On Wed, Nov 06, 2019 at 06:51:40PM +0300, Kirill Tkhai wrote:
>>>>>> +#ifdef CONFIG_SMP
>>>>>> +	if (!rq->nr_running) {
>>>>>> +		/*
>>>>>> +		 * Make sure task_on_rq_curr() fails, such that we don't do
>>>>>> +		 * put_prev_task() + set_next_task() on this task again.
>>>>>> +		 */
>>>>>> +		prev->on_cpu = 2;
>>>>>>  		newidle_balance(rq, rf);
>>>>>
>>>>> Shouldn't we restore prev->on_cpu = 1 after newidle_balance()? Can't prev
>>>>> become pickable again after newidle_balance() releases rq->lock, and we
>>>>> take it again, so this on_cpu == 2 never will be cleared?
>>>>
>>>> Indeed so.
>>>
>>> Oh wait, the way it was written this is not possible. Because
>>> rq->nr_running == 0 and prev->on_cpu > 0 it means the current task is
>>> going to sleep and cannot be woken back up.
>>
>> I mostly mean throttling. AFAIR, tasks of throttled rt_rq are not accounted
>> in rq->nr_running. But it seems rt_rq may become unthrottled again after
>> newidle_balance() unlocks rq lock, and prev will become pickable again.
> 
> Urgh... throttling.
> 
> Bah.. I had just named the "->on_cpu = 2" thing leave_task(), to match
> prepare_task() and finish_task(), but when we have to flip it back to 1
> that doesn't really work.
> 
> Also, I'm still flip-flopping on where to place it. Yesterday I
> suggested placing it before put_prev_task(), but then I went to write a
> comment, and either way around put_prev_task() needs to be very careful.
> 
> So I went back to placing it after and putting lots of comments on.
> 
> How does the below look?

For me the below patch looks OK.

One more thing about current code in git. After rq->lock became able to
be unlocked after put_prev_task() is commited, we got a new corner case.
We usually had the same order for running task:

  dequeue_task()
  put_prev_task()

Now the order may be reversed (this is also in case of throttling):

  put_prev_task() (called from pick_next_task())
  dequeue_task()  (called from another cpu)

This is more theoretically, since I don't see a problem here. But there are
too many statistics and counters in sched_class methods, that it is impossible
to be sure all of them work as expected.

Kirill

> ---
> Subject: sched: Fix pick_next_task() vs 'change' pattern race
> From: Peter Zijlstra <peterz@infradead.org>
> Date: Mon Nov 4 22:18:14 CET 2019
> 
> Commit 67692435c411 ("sched: Rework pick_next_task() slow-path")
> inadvertly introduced a race because it changed a previously
> unexplored dependency between dropping the rq->lock and
> sched_class::put_prev_task().
> 
> The comments about dropping rq->lock, in for example
> newidle_balance(), only mentions the task being current and ->on_cpu
> being set. But when we look at the 'change' pattern (in for example
> sched_setnuma()):
> 
> 	queued = task_on_rq_queued(p); /* p->on_rq == TASK_ON_RQ_QUEUED */
> 	running = task_current(rq, p); /* rq->curr == p */
> 
> 	if (queued)
> 		dequeue_task(...);
> 	if (running)
> 		put_prev_task(...);
> 
> 	/* change task properties */
> 
> 	if (queued)
> 		enqueue_task(...);
> 	if (running)
> 		set_next_task(...);
> 
> It becomes obvious that if we do this after put_prev_task() has
> already been called on @p, things go sideways. This is exactly what
> the commit in question allows to happen when it does:
> 
> 	prev->sched_class->put_prev_task(rq, prev, rf);
> 	if (!rq->nr_running)
> 		newidle_balance(rq, rf);
> 
> The newidle_balance() call will drop rq->lock after we've called
> put_prev_task() and that allows the above 'change' pattern to
> interleave and mess up the state.
> 
> The order in pick_next_task() is mandated by the fact that RT/DL
> put_prev_task() can pull other RT tasks, in which case we should not
> call newidle_balance() since we'll not be going idle. Similarly, we
> cannot put newidle_balance() in put_prev_task_fair() because it should
> be called every time we'll end up selecting the idle task.
> 
> Given that we're stuck with this order, the only solution is fixing
> the 'change' pattern. The simplest fix seems to be to 'absuse'
> p->on_cpu to carry more state. Adding more state to p->on_rq is
> possible but is far more invasive and also ends up duplicating much of
> the state we already carry in p->on_cpu.
> 
> Introduce task_on_rq_curr() to indicate the if
> sched_class::set_next_task() has been called -- and we thus need to
> call put_prev_task().
> 
> Fixes: 67692435c411 ("sched: Rework pick_next_task() slow-path")
> Reported-by: Quentin Perret <qperret@google.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Tested-by: Quentin Perret <qperret@google.com>
> Tested-by: Qais Yousef <qais.yousef@arm.com>
> Tested-by: Valentin Schneider <valentin.schneider@arm.com>
> Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>

Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>

> ---
>  kernel/sched/core.c     |   35 ++++++++++++++++++++++++++++-------
>  kernel/sched/deadline.c |    9 +++++++++
>  kernel/sched/rt.c       |    9 +++++++++
>  kernel/sched/sched.h    |   21 +++++++++++++++++++++
>  4 files changed, 67 insertions(+), 7 deletions(-)
> 
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1595,7 +1595,7 @@ void do_set_cpus_allowed(struct task_str
>  	lockdep_assert_held(&p->pi_lock);
>  
>  	queued = task_on_rq_queued(p);
> -	running = task_current(rq, p);
> +	running = task_on_rq_curr(rq, p);
>  
>  	if (queued) {
>  		/*
> @@ -3078,6 +3078,19 @@ static inline void prepare_task(struct t
>  #endif
>  }
>  
> +static inline void leave_task(struct task_struct *prev)
> +{
> +#ifdef CONFIG_SMP
> +	/*
> +	 * The task is on its way out, we'll have called put_prev_task() on it.
> +	 *
> +	 * Make sure task_on_rq_curr() fails, such that we don't do
> +	 * put_prev_task() + set_next_task() on this task again.
> +	 */
> +	prev->on_cpu = 2;
> +#endif
> +}
> +
>  static inline void finish_task(struct task_struct *prev)
>  {
>  #ifdef CONFIG_SMP
> @@ -3934,8 +3947,16 @@ pick_next_task(struct rq *rq, struct tas
>  	 * can PULL higher prio tasks when we lower the RQ 'priority'.
>  	 */
>  	prev->sched_class->put_prev_task(rq, prev, rf);
> -	if (!rq->nr_running)
> +	if (!rq->nr_running) {
> +		leave_task(prev);
>  		newidle_balance(rq, rf);
> +		/*
> +		 * When the below pick loop results in @p == @prev, then we
> +		 * will not go through context_switch() but the
> +		 * pick_next_task() will have done set_next_task() again.
> +		 */
> +		prepare_task(prev);
> +	}
>  
>  	for_each_class(class) {
>  		p = class->pick_next_task(rq, NULL, NULL);
> @@ -4422,7 +4443,7 @@ void rt_mutex_setprio(struct task_struct
>  
>  	prev_class = p->sched_class;
>  	queued = task_on_rq_queued(p);
> -	running = task_current(rq, p);
> +	running = task_on_rq_curr(rq, p);
>  	if (queued)
>  		dequeue_task(rq, p, queue_flag);
>  	if (running)
> @@ -4509,7 +4530,7 @@ void set_user_nice(struct task_struct *p
>  		goto out_unlock;
>  	}
>  	queued = task_on_rq_queued(p);
> -	running = task_current(rq, p);
> +	running = task_on_rq_curr(rq, p);
>  	if (queued)
>  		dequeue_task(rq, p, DEQUEUE_SAVE | DEQUEUE_NOCLOCK);
>  	if (running)
> @@ -4957,7 +4978,7 @@ static int __sched_setscheduler(struct t
>  	}
>  
>  	queued = task_on_rq_queued(p);
> -	running = task_current(rq, p);
> +	running = task_on_rq_curr(rq, p);
>  	if (queued)
>  		dequeue_task(rq, p, queue_flags);
>  	if (running)
> @@ -6141,7 +6162,7 @@ void sched_setnuma(struct task_struct *p
>  
>  	rq = task_rq_lock(p, &rf);
>  	queued = task_on_rq_queued(p);
> -	running = task_current(rq, p);
> +	running = task_on_rq_curr(rq, p);
>  
>  	if (queued)
>  		dequeue_task(rq, p, DEQUEUE_SAVE);
> @@ -7031,7 +7052,7 @@ void sched_move_task(struct task_struct
>  	rq = task_rq_lock(tsk, &rf);
>  	update_rq_clock(rq);
>  
> -	running = task_current(rq, tsk);
> +	running = task_on_rq_curr(rq, tsk);
>  	queued = task_on_rq_queued(tsk);
>  
>  	if (queued)
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1778,6 +1778,15 @@ pick_next_task_dl(struct rq *rq, struct
>  	return p;
>  }
>  
> +/*
> + * As per the note for sched_class::put_prev_task() we must only drop
> + * the rq->lock before any permanent state change.
> + *
> + * In this case, the state change and the pull action are mutually exclusive.
> + * If @prev is still on the runqueue, the priority will not have dropped and we
> + * don't need to pull. If @prev is no longer on the runqueue we don't need
> + * to add it back to the pushable list.
> + */
>  static void put_prev_task_dl(struct rq *rq, struct task_struct *p, struct rq_flags *rf)
>  {
>  	update_curr_dl(rq);
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1566,6 +1566,15 @@ pick_next_task_rt(struct rq *rq, struct
>  	return p;
>  }
>  
> +/*
> + * As per the note for sched_class::put_prev_task() we must only drop
> + * the rq->lock before any permanent state change.
> + *
> + * In this case, the state change and the pull action are mutually exclusive.
> + * If @prev is still on the runqueue, the priority will not have dropped and we
> + * don't need to pull. If @prev is no longer on the runqueue we don't need
> + * to add it back to the pushable list.
> + */
>  static void put_prev_task_rt(struct rq *rq, struct task_struct *p, struct rq_flags *rf)
>  {
>  	update_curr_rt(rq);
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1628,6 +1628,22 @@ static inline int task_running(struct rq
>  #endif
>  }
>  
> +/*
> + * If true, @p has had sched_class::set_next_task() called on it.
> + * See pick_next_task().
> + */
> +static inline bool task_on_rq_curr(struct rq *rq, struct task_struct *p)
> +{
> +#ifdef CONFIG_SMP
> +	return rq->curr == p && p->on_cpu == 1;
> +#else
> +	return rq->curr == p;
> +#endif
> +}
> +
> +/*
> + * If true, @p has has sched_class::enqueue_task() called on it.
> + */
>  static inline int task_on_rq_queued(struct task_struct *p)
>  {
>  	return p->on_rq == TASK_ON_RQ_QUEUED;
> @@ -1727,6 +1743,11 @@ struct sched_class {
>  	struct task_struct * (*pick_next_task)(struct rq *rq,
>  					       struct task_struct *prev,
>  					       struct rq_flags *rf);
> +	/*
> +	 * When put_prev_task() drops the rq->lock (RT/DL) it must do this
> +	 * before any effective state change, such that a nested
> +	 * 'put_prev_task() + set_next_task' pair will work correctly.
> +	 */
>  	void (*put_prev_task)(struct rq *rq, struct task_struct *p, struct rq_flags *rf);
>  	void (*set_next_task)(struct rq *rq, struct task_struct *p);
>  
>