On Monday 10 March 2008 23:31, 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. Hi, I can recreate a problem that looks very similar to this on a kernel based on 2.6.24.3-rt3. Hiroshi-san's patch seems to fix the problem for me. Ingo, is this patch going to be included in the next -rt patch? I see that it is already part of mainline. Thanks, Sripathi. > > --- > 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. > > Signed-off-by: Hiroshi Shimamoto <h-shimamoto@xxxxxxxxxxxxx> > --- > kernel/sched.c | 38 ++++++++++++++++---------------------- > 1 files changed, 16 insertions(+), 22 deletions(-) > > diff --git a/kernel/sched.c b/kernel/sched.c > index 52b9867..eedf748 100644 > --- a/kernel/sched.c > +++ b/kernel/sched.c > @@ -4268,11 +4268,10 @@ void rt_mutex_setprio(struct task_struct *p, > int prio) oldprio = p->prio; > on_rq = p->se.on_rq; > running = task_current(rq, p); > - if (on_rq) { > + if (on_rq) > dequeue_task(rq, p, 0); > - if (running) > - p->sched_class->put_prev_task(rq, p); > - } > + if (running) > + p->sched_class->put_prev_task(rq, p); > > if (rt_prio(prio)) > p->sched_class = &rt_sched_class; > @@ -4281,10 +4280,9 @@ void rt_mutex_setprio(struct task_struct *p, > int prio) > > p->prio = prio; > > + if (running) > + p->sched_class->set_curr_task(rq); > if (on_rq) { > - if (running) > - p->sched_class->set_curr_task(rq); > - > enqueue_task(rq, p, 0); > > check_class_changed(rq, p, prev_class, oldprio, running); > @@ -4581,19 +4579,17 @@ recheck: > update_rq_clock(rq); > on_rq = p->se.on_rq; > running = task_current(rq, p); > - if (on_rq) { > + if (on_rq) > deactivate_task(rq, p, 0); > - if (running) > - p->sched_class->put_prev_task(rq, p); > - } > + if (running) > + p->sched_class->put_prev_task(rq, p); > > oldprio = p->prio; > __setscheduler(rq, p, policy, param->sched_priority); > > + if (running) > + p->sched_class->set_curr_task(rq); > if (on_rq) { > - if (running) > - p->sched_class->set_curr_task(rq); > - > activate_task(rq, p, 0); > > check_class_changed(rq, p, prev_class, oldprio, running); > @@ -7617,11 +7613,10 @@ void sched_move_task(struct task_struct *tsk) > running = task_current(rq, tsk); > on_rq = tsk->se.on_rq; > > - if (on_rq) { > + if (on_rq) > dequeue_task(rq, tsk, 0); > - if (unlikely(running)) > - tsk->sched_class->put_prev_task(rq, tsk); > - } > + if (unlikely(running)) > + tsk->sched_class->put_prev_task(rq, tsk); > > set_task_rq(tsk, task_cpu(tsk)); > > @@ -7630,11 +7625,10 @@ void sched_move_task(struct task_struct *tsk) > tsk->sched_class->moved_group(tsk); > #endif > > - if (on_rq) { > - if (unlikely(running)) > - tsk->sched_class->set_curr_task(rq); > + if (unlikely(running)) > + tsk->sched_class->set_curr_task(rq); > + if (on_rq) > enqueue_task(rq, tsk, 0); > - } > > task_rq_unlock(rq, &flags); > } -- 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