sched: fix race in schedule()

Fix a hard to trigger crash seen in the -rt kernel that also affects
the vanilla scheduler.

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@ct.jp.nec.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/kernel/sched.c b/kernel/sched.c
index 1cb53fb1fe3ddcc102c9de1e44350e384fe66fff..9df9ba73cb7a941ea7a8feb18a58b35e726b26a4 100644 (file)
--- 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);
@@ -7618,11 +7614,10 @@ 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));
 
@@ -7631,11 +7626,10 @@ 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);
 }