Commit 6c6c54e1 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar

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: default avatarSergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1307115427.2353.3456.camel@twinsSigned-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent f2513cde
...@@ -605,10 +605,10 @@ static inline int cpu_of(struct rq *rq) ...@@ -605,10 +605,10 @@ static inline int cpu_of(struct rq *rq)
/* /*
* Return the group to which this tasks belongs. * Return the group to which this tasks belongs.
* *
* We use task_subsys_state_check() and extend the RCU verification * We use task_subsys_state_check() and extend the RCU verification with
* with lockdep_is_held(&p->pi_lock) because cpu_cgroup_attach() * pi->lock and rq->lock because cpu_cgroup_attach() holds those locks for each
* holds that lock for each task it moves into the cgroup. Therefore * task it moves into the cgroup. Therefore by holding either of those locks,
* by holding that lock, we pin the task to the current cgroup. * we pin the task to the current cgroup.
*/ */
static inline struct task_group *task_group(struct task_struct *p) 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) ...@@ -616,7 +616,8 @@ static inline struct task_group *task_group(struct task_struct *p)
struct cgroup_subsys_state *css; struct cgroup_subsys_state *css;
css = task_subsys_state_check(p, cpu_cgroup_subsys_id, 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); tg = container_of(css, struct task_group, css);
return autogroup_task_group(p, tg); return autogroup_task_group(p, tg);
...@@ -2200,6 +2201,16 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu) ...@@ -2200,6 +2201,16 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
!(task_thread_info(p)->preempt_count & PREEMPT_ACTIVE)); !(task_thread_info(p)->preempt_count & PREEMPT_ACTIVE));
#ifdef CONFIG_LOCKDEP #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) || WARN_ON_ONCE(debug_locks && !(lockdep_is_held(&p->pi_lock) ||
lockdep_is_held(&task_rq(p)->lock))); lockdep_is_held(&task_rq(p)->lock)));
#endif #endif
......
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