cpuset: lockdep spew on proc_cpuset_show

[Sasha Levin <sasha.levin@oracle.com>]

Hi Tejun,

I've stumbled on the following:

[   75.972016] ===============================
[   75.977317] [ INFO: suspicious RCU usage. ]
[   76.041031] 3.8.0-rc3-next-20130114-sasha-00016-ga107525-dirty #262 Tainted: G        W
[   76.057535] -------------------------------
[   76.063397] include/linux/cgroup.h:534 suspicious rcu_dereference_check() usage!
[   76.076333]
[   76.076333] other info that might help us debug this:
[   76.076333]
[   76.087091]
[   76.087091] rcu_scheduler_active = 1, debug_locks = 1
[   76.098682] 2 locks held by trinity/7514:
[   76.104154]  #0:  (&p->lock){+.+.+.}, at: [<ffffffff812b06aa>] seq_read+0x3a/0x3e0
[   76.119533]  #1:  (cpuset_mutex){+.+...}, at: [<ffffffff811abae4>] proc_cpuset_show+0x84/0x190
[   76.151167]
[   76.151167] stack backtrace:
[   76.156853] Pid: 7514, comm: trinity Tainted: G        W    3.8.0-rc3-next-20130114-sasha-00016-ga107525-dirty #262
[   76.180547] Call Trace:
[   76.183754]  [<ffffffff81182cab>] lockdep_rcu_suspicious+0x10b/0x120
[   76.191885]  [<ffffffff811abb71>] proc_cpuset_show+0x111/0x190
[   76.200572]  [<ffffffff812b0827>] seq_read+0x1b7/0x3e0
[   76.206843]  [<ffffffff812b0670>] ? seq_lseek+0x110/0x110
[   76.213562]  [<ffffffff8128b4fb>] do_loop_readv_writev+0x4b/0x90
[   76.220961]  [<ffffffff8128b776>] do_readv_writev+0xf6/0x1d0
[   76.227940]  [<ffffffff8128b8ee>] vfs_readv+0x3e/0x60
[   76.235971]  [<ffffffff8128b960>] sys_readv+0x50/0xd0
[   76.241945]  [<ffffffff83d33d18>] tracesys+0xe1/0xe6

This is the result of "cpuset: replace cgroup_mutex locking with cpuset internal locking"
which remove the cgroup_lock() before calling task_subsys_state(), which now complains since
one of the debug conditions in the rcu_dereference_check() there is cgroup_lock_is_held().

I'm not sure if this is an actual issue, but I do see that task_subsys_state() gets called
from other places as well, so I think that possibly lockdep is right here and we do need
to fix up locking there, but again, I'm not sure...


Thanks,
Sasha