with CONFIG_PROVE_RCU=y, I saw this warning, it's because
css_id() is not under rcu_read_lock().
===================================================
[ INFO: suspicious rcu_dereference_check() usage. ]
---------------------------------------------------
kernel/cgroup.c:4438 invoked rcu_dereference_check() without protection!
other info that might help us debug this:
rcu_scheduler_active = 1, debug_locks = 1
1 lock held by kswapd0/31:
#0: (swap_lock){+.+.-.}, at: [<c05058bb>] swap_info_get+0x4b/0xd0
stack backtrace:
Pid: 31, comm: kswapd0 Not tainted 2.6.34-rc5-tip+ #13
Call Trace:
[<c083c5d6>] ? printk+0x1d/0x1f
[<c0480744>] lockdep_rcu_dereference+0x94/0xb0
[<c049d6ed>] css_id+0x5d/0x60
[<c05165a5>] mem_cgroup_uncharge_swapcache+0x45/0xa0
[<c0505e4f>] swapcache_free+0x3f/0x60
[<c04e79e2>] __remove_mapping+0xb2/0xf0
[<c04e7cbb>] shrink_page_list+0x26b/0x490
[<c047f85d>] ? put_lock_stats+0xd/0x30
[<c083fd67>] ? _raw_spin_unlock_irq+0x27/0x50
[<c0482566>] ? trace_hardirqs_on_caller+0xb6/0x220
[<c04e8158>] shrink_inactive_list+0x278/0x620
[<c04729e1>] ? sched_clock_cpu+0x121/0x180
[<c047e9b8>] ? trace_hardirqs_off_caller+0x18/0x130
[<c047eadb>] ? trace_hardirqs_off+0xb/0x10
[<c0843438>] ? sub_preempt_count+0x8/0x90
[<c047f85d>] ? put_lock_stats+0xd/0x30
[<c04e8704>] shrink_zone+0x204/0x3c0
[<c083fcac>] ? _raw_spin_unlock+0x2c/0x50
[<c04e951e>] kswapd+0x61e/0x7c0
[<c04e6ed0>] ? isolate_pages_global+0x0/0x1f0
[<c046bae0>] ? autoremove_wake_function+0x0/0x50
[<c04e8f00>] ? kswapd+0x0/0x7c0
[<c046b5e4>] kthread+0x74/0x80
[<c046b570>] ? kthread+0x0/0x80
[<c04035ba>] kernel_thread_helper+0x6/0x10
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