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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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