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SubjectRe: [BUG: NULL pointer dereference] cgroups and RT scheduling interact badly.
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On Mon, 2008-06-16 at 17:18 +0200, Peter Zijlstra wrote:
> On Mon, 2008-06-16 at 17:11 +0200, Daniel K. wrote:
> > Peter Zijlstra wrote:
> > > On Mon, 2008-06-16 at 15:51 +0200, Peter Zijlstra wrote:
> > >> On Mon, 2008-06-16 at 15:14 +0200, Daniel K. wrote:
> > >>> Peter Zijlstra wrote:
> > >>>
> > >>> Although this patch seems to be correct, this is what shows up on my
> > >>> netconsole, when applying it -- with an offset, do you have other fixes
> > >>> applied as well?
> > >> I had indeed, although nothing touching the rt scheduler. I popped all
> > >> my patches and pulled an update from Linus, but I fail to reproduce the
> > >> below.
> > >>
> > >> /me goes look for that burnp6 thing, I used a simple while (1); loop.
> > >
> > > found it, still seems to work for me. do you have a funny number of
> > > cpus? or anything else noteworthy?
> >
> > I don't think so, this is on a SUN X2200 M2, with two AMD Opteron 2214
> > processors, and 8G RAM.
> >
> > If I follow the procedure up to 'echo 4000 > oops/cpu.rt_runtime_us'
> > then I can
> >
> > # burnP6 &
> > [1] 3395
> > # schedtool -R -p 1 3395
> >
> > but
> >
> > # echo -n 3395 > /dev/cgroup/burn/oops/tasks
>
> Ah, that did it,.. I'll go poke at it. Thanks!

How's this work for you? (includes the previuos patchlet too)

---
Subject: sched: rt-group: fix NULL deref

- the rt group hierarchy got corrupted by always pointing the entity's
rq pointer to the root rq.

- the enqueue/dequeue on throttle should also use the full
dequeue_rt_stack like regular task enqueue/dequeue do. Not doing this
can leave empty groups and possibly corrupt the priority queues.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
diff --git a/kernel/sched.c b/kernel/sched.c
index eaf6751..efbb7d9 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -7626,7 +7626,6 @@ static void init_tg_rt_entry(struct task_group *tg, struct rt_rq *rt_rq,
else
rt_se->rt_rq = parent->my_q;

- rt_se->rt_rq = &rq->rt;
rt_se->my_q = rt_rq;
rt_se->parent = parent;
INIT_LIST_HEAD(&rt_se->run_list);
diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index 3432d57..95c408e 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -449,7 +449,7 @@ void dec_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
#endif
}

-static void enqueue_rt_entity(struct sched_rt_entity *rt_se)
+static void __enqueue_rt_entity(struct sched_rt_entity *rt_se)
{
struct rt_rq *rt_rq = rt_rq_of_se(rt_se);
struct rt_prio_array *array = &rt_rq->active;
@@ -464,7 +464,7 @@ static void enqueue_rt_entity(struct sched_rt_entity *rt_se)
inc_rt_tasks(rt_se, rt_rq);
}

-static void dequeue_rt_entity(struct sched_rt_entity *rt_se)
+static void __dequeue_rt_entity(struct sched_rt_entity *rt_se)
{
struct rt_rq *rt_rq = rt_rq_of_se(rt_se);
struct rt_prio_array *array = &rt_rq->active;
@@ -480,11 +480,10 @@ static void dequeue_rt_entity(struct sched_rt_entity *rt_se)
* Because the prio of an upper entry depends on the lower
* entries, we must remove entries top - down.
*/
-static void dequeue_rt_stack(struct task_struct *p)
+static void dequeue_rt_stack(struct sched_rt_entity *rt_se)
{
- struct sched_rt_entity *rt_se, *back = NULL;
+ struct sched_rt_entity *back = NULL;

- rt_se = &p->rt;
for_each_sched_rt_entity(rt_se) {
rt_se->back = back;
back = rt_se;
@@ -492,7 +491,26 @@ static void dequeue_rt_stack(struct task_struct *p)

for (rt_se = back; rt_se; rt_se = rt_se->back) {
if (on_rt_rq(rt_se))
- dequeue_rt_entity(rt_se);
+ __dequeue_rt_entity(rt_se);
+ }
+}
+
+static void enqueue_rt_entity(struct sched_rt_entity *rt_se)
+{
+ dequeue_rt_stack(rt_se);
+ for_each_sched_rt_entity(rt_se)
+ __enqueue_rt_entity(rt_se);
+}
+
+static void dequeue_rt_entity(struct sched_rt_entity *rt_se)
+{
+ dequeue_rt_stack(rt_se);
+
+ for_each_sched_rt_entity(rt_se) {
+ struct rt_rq *rt_rq = group_rt_rq(rt_se);
+
+ if (rt_rq && rt_rq->rt_nr_running)
+ __enqueue_rt_entity(rt_se);
}
}

@@ -506,32 +524,15 @@ static void enqueue_task_rt(struct rq *rq, struct task_struct *p, int wakeup)
if (wakeup)
rt_se->timeout = 0;

- dequeue_rt_stack(p);
-
- /*
- * enqueue everybody, bottom - up.
- */
- for_each_sched_rt_entity(rt_se)
- enqueue_rt_entity(rt_se);
+ enqueue_rt_entity(rt_se);
}

static void dequeue_task_rt(struct rq *rq, struct task_struct *p, int sleep)
{
struct sched_rt_entity *rt_se = &p->rt;
- struct rt_rq *rt_rq;

update_curr_rt(rq);
-
- dequeue_rt_stack(p);
-
- /*
- * re-enqueue all non-empty rt_rq entities.
- */
- for_each_sched_rt_entity(rt_se) {
- rt_rq = group_rt_rq(rt_se);
- if (rt_rq && rt_rq->rt_nr_running)
- enqueue_rt_entity(rt_se);
- }
+ dequeue_rt_entity(rt_se);
}

/*



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