On Mon, 2008-03-10 at 11:01 -0700, Hiroshi Shimamoto wrote: > Hi Ingo, > > I found a race condition in scheduler. > The first report is the below; > http://lkml.org/lkml/2008/2/26/459 > > It took a bit long time to investigate and I couldn't have much time last week. > It is hard to reproduce but -rt is little easier because it has preemptible > spin lock and rcu. > > Could you please check the scenario and the patch. > It will be needed for the stable, too. > > --- > From: Hiroshi Shimamoto <h-shimamoto@xxxxxxxxxxxxx> > > There is a race condition between schedule() and some dequeue/enqueue > functions; rt_mutex_setprio(), __setscheduler() and sched_move_task(). > > When scheduling to idle, idle_balance() is called to pull tasks from > other busy processor. It might drop the rq lock. > It means that those 3 functions encounter on_rq=0 and running=1. > The current task should be put when running. > > Here is a possible scenario; > CPU0 CPU1 > | schedule() > | ->deactivate_task() > | ->idle_balance() > | -->load_balance_newidle() > rt_mutex_setprio() | > | --->double_lock_balance() > *get lock *rel lock > * on_rq=0, ruuning=1 | > * sched_class is changed | > *rel lock *get lock > : | > : > ->put_prev_task_rt() > ->pick_next_task_fair() > => panic > > The current process of CPU1(P1) is scheduling. Deactivated P1, > and the scheduler looks for another process on other CPU's runqueue > because CPU1 will be idle. idle_balance(), load_balance_newidle() > and double_lock_balance() are called and double_lock_balance() could > drop the rq lock. On the other hand, CPU0 is trying to boost the > priority of P1. The result of boosting only P1's prio and sched_class > are changed to RT. The sched entities of P1 and P1's group are never > put. It makes cfs_rq invalid, because the cfs_rq has curr and no leaf, > but pick_next_task_fair() is called, then the kernel panics. Very nice catch, this had me puzzled for a while. I'm not quite sure I fully understand. Could you explain why the below isn't sufficient? --- diff --git a/kernel/sched.c b/kernel/sched.c index a0c79e9..ebd9fc5 100644 --- a/kernel/sched.c +++ b/kernel/sched.c @@ -4067,10 +4067,11 @@ need_resched_nonpreemptible: prev->sched_class->pre_schedule(rq, prev); #endif + prev->sched_class->put_prev_task(rq, prev); + if (unlikely(!rq->nr_running)) idle_balance(cpu, rq); - prev->sched_class->put_prev_task(rq, prev); next = pick_next_task(rq, prev); sched_info_switch(prev, next); -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
