On 2016/07/13 at 09:50, Wanpeng Li wrote: > 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? cfs_rq_throttled() returns false for child cgroups in the throttled hierarchy, so throttled_hierarchy() should be relied on in such cases. Regards, Xunlei -- 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