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Subject[PATCH] sched, autogroup: Fix failure when writing to cpu.rt_runtime_us
This is how to reproduce the bug:

int main() {
struct sched_param param = {.sched_priority=1};

if (fork() > 0)
exit(0);

setsid();

if (sched_setscheduler(0, SCHED_RR, &param) < 0){
perror("failed to sched_setscheduler()");
return -1;
}

while(1)
;
}

# ./test
# mount -t cgroup -o cpu xxx /cgroup
# cat /cgroup/cpu.rt_runtime_us
950000
# echo 940000 > /cgroup/cpu.rt_runtime_us
Device or Resource busy

An autogroup has been created and there's an RT task in it. RT tasks in
autogroups are redirected to the root group and task_group() should
return &root_task_group, but it's broken and returns the autogroup.

We should reset p->sched_task_group when changing a normal task to an
RT task which is in autogroup.

Fixes: 8323f26ce342 ("sched: Fix race in task_group()")
Cc: <stable@vger.kernel.org> # 3.6+
Reported-by: Zhang Wei <zhangwei555@huawei.com>
Signed-off-by: Zefan Li <lizefan@huawei.com>
---
kernel/sched/core.c | 15 +++++++++++----
1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 89e7283..fccde96 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3414,6 +3414,7 @@ static int __sched_setscheduler(struct task_struct *p,
const struct sched_class *prev_class;
struct rq *rq;
int reset_on_fork;
+ bool reset_task_group = false;

/* may grab non-irq protected spin_locks */
BUG_ON(in_interrupt());
@@ -3546,10 +3547,13 @@ change:
* assigned.
*/
if (rt_bandwidth_enabled() && rt_policy(policy) &&
- task_group(p)->rt_bandwidth.rt_runtime == 0 &&
- !task_group_is_autogroup(task_group(p))) {
- task_rq_unlock(rq, p, &flags);
- return -EPERM;
+ task_group(p)->rt_bandwidth.rt_runtime == 0) {
+ if (!task_group_is_autogroup(task_group(p))) {
+ task_rq_unlock(rq, p, &flags);
+ return -EPERM;
+ } else {
+ reset_task_group = true;
+ }
}
#endif
#ifdef CONFIG_SMP
@@ -3615,6 +3619,9 @@ change:
prev_class = p->sched_class;
__setscheduler(rq, p, attr);

+ if (reset_task_group)
+ p->sched_task_group = &root_task_group;
+
if (running)
p->sched_class->set_curr_task(rq);
if (queued) {
--
1.8.0.2


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