Messages in this thread | | | Subject | Re: NULL pointer dereference in pick_next_task_fair | From | Kirill Tkhai <> | Date | Thu, 7 Nov 2019 18:12:07 +0300 |
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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); > >
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