[PATCH] sched: Fix race in task_group()

[Peter Zijlstra]

Stefan reported a crash on a kernel before a3e5d1091c1 ("sched: Don't
call task_group() too many times in set_task_rq()"), he found the reason
to be that the multiple task_group() invocations in set_task_rq()
returned different values.

Looking at all that I found a lack of serialization and plain wrong
comments.

The below tries to fix it using an extra pointer which is updated under
the appropriate scheduler locks. Its not pretty, but I can't really see
another way given how all the cgroup stuff works.

Reported-and-tested-by: Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
[backported to apply to 3.0 and 3.2]
Signed-off-by: Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
---
 include/linux/init_task.h |   12 +++++++++++-
 include/linux/sched.h     |    5 ++++-
 kernel/sched.c            |   32 ++++++++++++++++++--------------
 3 files changed, 33 insertions(+), 16 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 56de5c1..1fd9884 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1242,6 +1242,9 @@ struct task_struct {
 	const struct sched_class *sched_class;
 	struct sched_entity se;
 	struct sched_rt_entity rt;
+#ifdef CONFIG_CGROUP_SCHED
+	struct task_struct *sched_task_group;
+#endif

where sched_task_group was defined to be task_struct twice(in the first
and the third versions) and to be task_group once.

Before backport, feel free to respin with the final define determined.
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