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sched: Fix race in task_group()

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Stefan reported a crash on a kernel before a3e5d1091c ("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@canonical.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1340364965.18025.71.camel@twins
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra
2012-06-22 13:36:05 +02:00
committed by Ingo Molnar
parent 88b8dac0a1
commit 8323f26ce3
4 changed files with 33 additions and 16 deletions
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23
kernel/sched/sched.h
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@@ -538,22 +538,19 @@ extern int group_balance_cpu(struct sched_group *sg);
/*
* Return the group to which this tasks belongs.
*
* We use task_subsys_state_check() and extend the RCU verification with
* pi->lock and rq->lock because cpu_cgroup_attach() holds those locks for each
* task it moves into the cgroup. Therefore by holding either of those locks,
* we pin the task to the current cgroup.
* We cannot use task_subsys_state() and friends because the cgroup
* subsystem changes that value before the cgroup_subsys::attach() method
* is called, therefore we cannot pin it and might observe the wrong value.
*
* The same is true for autogroup's p->signal->autogroup->tg, the autogroup
* core changes this before calling sched_move_task().
*
* Instead we use a 'copy' which is updated from sched_move_task() while
* holding both task_struct::pi_lock and rq::lock.
*/
static inline struct task_group *task_group(struct task_struct *p)
{
struct task_group *tg;
struct cgroup_subsys_state *css;
css = task_subsys_state_check(p, cpu_cgroup_subsys_id,
lockdep_is_held(&p->pi_lock) ||
lockdep_is_held(&task_rq(p)->lock));
tg = container_of(css, struct task_group, css);
return autogroup_task_group(p, tg);
return p->sched_task_group;
}
/* Change a task's cfs_rq and parent entity if it moves across CPUs/groups */