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    Subject[PATCH 3.12 75/76] sched: Remove lockdep check in sched_move_task()
    Date
    From: Kirill Tkhai <ktkhai@parallels.com>

    3.12-stable review patch. If anyone has any objections, please let me know.

    ===============

    commit f7b8a47da17c9ee4998f2ca2018fcc424e953c0e upstream.

    sched_move_task() is the only interface to change sched_task_group:
    cpu_cgrp_subsys methods and autogroup_move_group() use it.

    Everything is synchronized by task_rq_lock(), so cpu_cgroup_attach()
    is ordered with other users of sched_move_task(). This means we do no
    need RCU here: if we've dereferenced a tg here, the .attach method
    hasn't been called for it yet.

    Thus, we should pass "true" to task_css_check() to silence lockdep
    warnings.

    Fixes: eeb61e53ea19 ("sched: Fix race between task_group and sched_task_group")
    Reported-by: Oleg Nesterov <oleg@redhat.com>
    Reported-by: Fengguang Wu <fengguang.wu@intel.com>
    Signed-off-by: Kirill Tkhai <ktkhai@parallels.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Link: http://lkml.kernel.org/r/1414473874.8574.2.camel@tkhai
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Signed-off-by: Jiri Slaby <jslaby@suse.cz>
    ---
    kernel/sched/core.c | 8 ++++++--
    1 file changed, 6 insertions(+), 2 deletions(-)

    diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    index dd794a9b6850..e382c14652d0 100644
    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -6865,8 +6865,12 @@ void sched_move_task(struct task_struct *tsk)
    if (unlikely(running))
    tsk->sched_class->put_prev_task(rq, tsk);

    - tg = container_of(task_css_check(tsk, cpu_cgroup_subsys_id,
    - lockdep_is_held(&tsk->sighand->siglock)),
    + /*
    + * All callers are synchronized by task_rq_lock(); we do not use RCU
    + * which is pointless here. Thus, we pass "true" to task_css_check()
    + * to prevent lockdep warnings.
    + */
    + tg = container_of(task_css_check(tsk, cpu_cgroup_subsys_id, true),
    struct task_group, css);
    tg = autogroup_task_group(tsk, tg);
    tsk->sched_task_group = tg;
    --
    2.8.2
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