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. In the other order (set_next_buddy then throttle), throttle_cfs_rq will call dequeue which will clear the problematic buddy. -- 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