Messages in this thread | | | Subject | Re: Re: NULL pointer dereference in pick_next_task_fair | From | Kirill Tkhai <> | Date | Wed, 6 Nov 2019 18:51:40 +0300 |
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On 06.11.2019 15:05, Peter Zijlstra wrote: > On Mon, Oct 28, 2019 at 05:46:03PM +0000, Quentin Perret wrote: >> >> After digging a bit, the offending commit seems to be: >> >> 67692435c411 ("sched: Rework pick_next_task() slow-path") >> >> By 'offending' I mean that reverting it makes the issue go away. The >> issue comes from the fact that pick_next_entity() returns a NULL se in >> the 'simple' path of pick_next_task_fair(), which causes obvious >> problems in the subsequent call to set_next_entity(). >> >> I'll dig more, but if anybody understands the issue in the meatime feel >> free to send me a patch to try out :) > > So for all those who didn't follow along on IRC, the below seems to cure > things. > > The only thing I'm now considering is if we shouldn't be setting > ->on_cpu=2 _before_ calling put_prev_task(). I'll go audit the RT/DL > cases. > > --- > 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> > --- > kernel/sched/core.c | 22 +++++++++++++++------- > kernel/sched/sched.h | 16 ++++++++++++++++ > 2 files changed, 31 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) { > /* > @@ -3934,8 +3934,16 @@ 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) > +#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?
> + } > +#endif > > for_each_class(class) { > p = class->pick_next_task(rq, NULL, NULL); > @@ -4422,7 +4430,7 @@ 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 +4517,7 @@ 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 +4965,7 @@ 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 +6149,7 @@ 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 +7039,7 @@ 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/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; >
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