2016-07-15 6:49 GMT+08:00 <bsegall@xxxxxxxxxx>: > Wanpeng Li <kernellwp@xxxxxxxxx> writes: > >> 2016-07-15 1:54 GMT+08:00 <bsegall@xxxxxxxxxx>: >>> Wanpeng Li <kernellwp@xxxxxxxxx> writes: >>> >>>> 2016-07-14 1:06 GMT+08:00 <bsegall@xxxxxxxxxx>: >>>>> Wanpeng Li <kernellwp@xxxxxxxxx> writes: >>>>> >>>>>> 2016-07-13 1:25 GMT+08:00 <bsegall@xxxxxxxxxx>: >>>>>>> Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx> writes: >>>>>>> >>>>>>>> On 11.07.2016 15:12, Xunlei Pang wrote: >>>>>>>>> On 2016/07/11 at 17:54, Wanpeng Li wrote: >>>>>>>>>> Hi Konstantin, Xunlei, >>>>>>>>>> 2016-07-11 16:42 GMT+08:00 Xunlei Pang <xpang@xxxxxxxxxx>: >>>>>>>>>>> On 2016/07/11 at 16:22, Xunlei Pang wrote: >>>>>>>>>>>> On 2016/07/11 at 15:25, Wanpeng Li wrote: >>>>>>>>>>>>> 2016-06-16 20:57 GMT+08:00 Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>: >>>>>>>>>>>>>> Hierarchy could be already throttled at this point. Throttled next >>>>>>>>>>>>>> buddy could trigger null pointer dereference in pick_next_task_fair(). >>>>>>>>>>>>> There is cfs_rq->next check in pick_next_entity(), so how can null >>>>>>>>>>>>> pointer dereference happen? >>>>>>>>>>>> I guess it's the following code leading to a NULL se returned: >>>>>>>>>>> s/NULL/empty-entity cfs_rq se/ >>>>>>>>>>> >>>>>>>>>>>> pick_next_entity(): >>>>>>>>>>>> if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1) >>>>>>>>>> ^^^^^^^^^^^^^ >>>>>>>>>> I think this will return false. >>>>>>>>> >>>>>>>>> With the wrong throttled_hierarchy(), I think this can happen. But after we have the >>>>>>>>> corrected throttled_hierarchy() patch, I can't see how it is possible. >>>>>>>>> >>>>>>>>> dequeue_task_fair(): >>>>>>>>> if (task_sleep && parent_entity(se)) >>>>>>>>> set_next_buddy(parent_entity(se)); >>>>>>>>> >>>>>>>>> How does dequeue_task_fair() with DEQUEUE_SLEEP set(true task_sleep) happen to a throttled hierarchy? >>>>>>>>> IOW, a task belongs to a throttled hierarchy is running? >>>>>>>>> >>>>>>>>> Maybe Konstantin knows the reason. >>>>>>>> >>>>>>>> This function (dequeue_task_fair) check throttling but at point it could skip several >>>>>>>> levels and announce as next buddy actually throttled entry. >>>>>>>> Probably this bug hadn't happened but this's really hard to prove that this is impossible. >>>>>>>> ->set_curr_task(), PI-boost or some tricky migration in balancer could break this easily. >>>>>>> >>>>>>> sched_setscheduler can call put_prev_task, which then can cause a >>>>>>> throttle outside of __schedule(), then the task blocks normally and >>>>>>> deactivate_task(DEQUEUE_SLEEP) happens and you lose. >>>>>> >>>>>> The cfs_rq_throttled() check in dequeue_task_fair() will capture the >>>>>> cfs_rq which is throttled in sched_setscheduler::put_prev_task path, >>>>>> so nothing lost, where I miss? >>>>>> >>>>>> Regards, >>>>>> Wanpeng Li >>>>> >>>>> The cfs_rq_throttled() checks there are done bottom-up, so they will >>>>> trigger too late. a/b/t, where t is descheduling and a is throttled can >>>>> still cause a set_next_buddy(b); >>>> >>>> throttle cfs_rq is up-bottom, so when a is throttled, b and c are not >>>> yet, then task_sleep && se && !throttled_hierarchy(cfs_rq) still can't >>>> prevent a set_next_buddy(b). >>>> >>>> Regards, >>>> Wanpeng Li >>> >>> They don't race or anything, everything's under rq->lock. >>> throttled_hierarchy will register properly, the issue is that a parent >>> is the one cfs_rq_throttled(), not the current cfs_rq, and >>> set_next_buddy will set cfs_rq->next to an se that is !on_rq. >> >> Why b is !on_rq after throttle a? >> >> Regards, >> Wanpeng Li > > a is !on_rq (because of throttle), but set_next_buddy will set ->next up > the entire tree. Got it, thanks for your explanation. :) Regards, Wanpeng Li -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html