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Subject[tip:sched/urgent] sched: Fix/clarify set_task_cpu() locking rules
Commit-ID:  6c6c54e1807faf116724451ef2bd14993780470a
Gitweb: http://git.kernel.org/tip/6c6c54e1807faf116724451ef2bd14993780470a
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Fri, 3 Jun 2011 17:37:07 +0200
Committer: Ingo Molnar <mingo@elte.hu>
CommitDate: Tue, 7 Jun 2011 12:26:40 +0200

sched: Fix/clarify set_task_cpu() locking rules

Sergey reported a CONFIG_PROVE_RCU warning in push_rt_task where
set_task_cpu() was called with both relevant rq->locks held, which
should be sufficient for running tasks since holding its rq->lock
will serialize against sched_move_task().

Update the comments and fix the task_group() lockdep test.

Reported-and-tested-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1307115427.2353.3456.camel@twins
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
kernel/sched.c | 21 ++++++++++++++++-----
1 files changed, 16 insertions(+), 5 deletions(-)
diff --git a/kernel/sched.c b/kernel/sched.c
index 2fe98ed..3f2e502 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -605,10 +605,10 @@ static inline int cpu_of(struct rq *rq)
/*
* Return the group to which this tasks belongs.
*
- * We use task_subsys_state_check() and extend the RCU verification
- * with lockdep_is_held(&p->pi_lock) because cpu_cgroup_attach()
- * holds that lock for each task it moves into the cgroup. Therefore
- * by holding that lock, we pin the task to the current cgroup.
+ * 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.
*/
static inline struct task_group *task_group(struct task_struct *p)
{
@@ -616,7 +616,8 @@ static inline struct task_group *task_group(struct task_struct *p)
struct cgroup_subsys_state *css;

css = task_subsys_state_check(p, cpu_cgroup_subsys_id,
- lockdep_is_held(&p->pi_lock));
+ 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);
@@ -2200,6 +2201,16 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
!(task_thread_info(p)->preempt_count & PREEMPT_ACTIVE));

#ifdef CONFIG_LOCKDEP
+ /*
+ * The caller should hold either p->pi_lock or rq->lock, when changing
+ * a task's CPU. ->pi_lock for waking tasks, rq->lock for runnable tasks.
+ *
+ * sched_move_task() holds both and thus holding either pins the cgroup,
+ * see set_task_rq().
+ *
+ * Furthermore, all task_rq users should acquire both locks, see
+ * task_rq_lock().
+ */
WARN_ON_ONCE(debug_locks && !(lockdep_is_held(&p->pi_lock) ||
lockdep_is_held(&task_rq(p)->lock)));
#endif

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