rcu warning caused by [sched: Fix race between task_group and sched_task_group]

[Heiko Carstens]

Hi Kirill,

the problem below has already been reported here:
https://lkml.org/lkml/2014/11/11/505

but seems there is still no fix available upstream?
Would be good to have it soon, since this basically renders lockdep unusable.

[    0.173906] ===============================
[    0.173907] [ INFO: suspicious RCU usage. ]
[    0.173909] 3.18.0-rc5 #15 Not tainted
[    0.173909] -------------------------------
[    0.173910] kernel/sched/core.c:7449 suspicious rcu_dereference_check() usage!
[    0.173912] 
               other info that might help us debug this:
[    0.173913] 
               rcu_scheduler_active = 1, debug_locks = 0
[    0.173914] 2 locks held by swapper/0/0:
[    0.173915]  #0:  (&p->pi_lock){......}, at: [<000000000017baae>] task_rq_lock+0x4e/0xf8
[    0.173925]  #1:  (&rq->lock){......}, at: [<000000000017bad6>] task_rq_lock+0x76/0xf8
[    0.173928] 
[    0.173930] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 3.18.0-rc5 #15
[    0.173931]        0000000000b9fb70 0000000000b9fb80 0000000000000002 0000000000000000 
                      0000000000b9fc10 0000000000b9fb88 0000000000b9fb88 0000000000112638 
                      0000000000000000 00000000009d5b8c 00000000009be240 000000000000000b 
                      0000000000b9fbd0 0000000000b9fb70 0000000000000000 0000000000000000 
                      0000000000000000 0000000000112638 0000000000b9fb70 0000000000b9fbd0 
[    0.173944] Call Trace:
[    0.173947] ([<000000000011251c>] show_trace+0xf4/0x158)
[    0.173949]  [<00000000001125ea>] show_stack+0x6a/0xe8
[    0.173952]  [<0000000000807ac2>] dump_stack+0x82/0xb0
[    0.173955]  [<000000000018700a>] sched_move_task+0x20a/0x210
[    0.173959]  [<0000000000203bf0>] cgroup_post_fork+0x60/0x148
[    0.173963]  [<000000000014c5b8>] copy_process.part.46+0xfe8/0x1bc8
[    0.173964]  [<000000000014d39c>] do_fork+0x104/0x7d8
[    0.173965]  [<000000000014dab6>] kernel_thread+0x46/0x58
[    0.173968]  [<00000000007fe890>] rest_init+0x38/0x170
[    0.173971]  [<0000000000c45b40>] start_kernel+0x460/0x470
[    0.173972]  [<0000000000100020>] _stext+0x20/0x80
[    0.173973] INFO: lockdep is turned off.

caused by (bisected):

commit eeb61e53ea19be0c4015b00b2e8b3b2185436f2b
Author: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
Date:   Mon Oct 27 14:18:25 2014 +0400

    sched: Fix race between task_group and sched_task_group

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