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 -- 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