sched: Remove lockdep check in sched_move_task()

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>

1 files changed, 6 insertions, 2 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 240157c13ddc..6841fb46eb07 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7444,8 +7444,12 @@ void sched_move_task(struct task_struct *tsk)
         if (unlikely(running))
                 put_prev_task(rq, tsk);
 
-        tg = container_of(task_css_check(tsk, cpu_cgrp_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_cgrp_id, true),
                           struct task_group, css);
         tg = autogroup_task_group(tsk, tg);
         tsk->sched_task_group = tg;