perf related lockdep bug

[Dave Jones <davej@codemonkey.org.uk>]

This is new to me since todays merge window.

	Dave

[  293.955261] ======================================================
[  293.955395] [ INFO: possible circular locking dependency detected ]
[  293.955536] 4.3.0-think+ #3 Not tainted
[  293.955618] -------------------------------------------------------
[  293.955756] trinity-c343/12586 is trying to acquire lock:
[  293.955875]  (rcu_node_0){-.-...}, at: [<ffffffffb00eb734>] rcu_report_exp_cpu_mult+0x24/0x70
[  293.956070] 
               but task is already holding lock:
[  293.956194]  (&ctx->lock){-.....}, at: [<ffffffffb0180453>] perf_lock_task_context+0x53/0x300
[  293.956386] 
               which lock already depends on the new lock.

[  293.956559] 
               the existing dependency chain (in reverse order) is:
[  293.956720] 
               -> #3 (&ctx->lock){-.....}:
[  293.956807]        [<ffffffffb00d53a3>] lock_acquire+0xc3/0x1d0
[  293.956940]        [<ffffffffb07efdd1>] _raw_spin_lock+0x41/0x80
[  293.957075]        [<ffffffffb0185b66>] __perf_event_task_sched_out+0x276/0x700
[  293.957239]        [<ffffffffb07e9f70>] __schedule+0x530/0xaf0
[  293.957370]        [<ffffffffb07ea6df>] schedule+0x3f/0x90
[  293.957493]        [<ffffffffb0001206>] exit_to_usermode_loop+0x76/0xa0
[  293.957642]        [<ffffffffb0001c13>] prepare_exit_to_usermode+0x53/0x60
[  293.957795]        [<ffffffffb07f1867>] retint_user+0x8/0x20
[  293.957923] 
               -> #2 (&rq->lock){-.-.-.}:
[  293.958008]        [<ffffffffb00d53a3>] lock_acquire+0xc3/0x1d0
[  293.958140]        [<ffffffffb07efdd1>] _raw_spin_lock+0x41/0x80
[  293.958275]        [<ffffffffb00b198d>] sched_move_task+0x4d/0x270
[  293.958417]        [<ffffffffb00b1bde>] cpu_cgroup_fork+0xe/0x10
[  293.958551]        [<ffffffffb012a057>] cgroup_post_fork+0x67/0x160
[  293.958692]        [<ffffffffb007ae0a>] copy_process+0x147a/0x1e10
[  293.958831]        [<ffffffffb007b927>] _do_fork+0x97/0x680
[  293.958955]        [<ffffffffb007bf39>] kernel_thread+0x29/0x30
[  293.959087]        [<ffffffffb07e8477>] rest_init+0x27/0x140
[  293.959211]        [<ffffffffb0ef1f72>] start_kernel+0x450/0x45d
[  293.959347]        [<ffffffffb0ef12fe>] x86_64_start_reservations+0x2a/0x2c
[  293.959503]        [<ffffffffb0ef1468>] x86_64_start_kernel+0x168/0x176
[  293.959652] 
               -> #1 (&p->pi_lock){-.-.-.}:
[  293.959742]        [<ffffffffb00d53a3>] lock_acquire+0xc3/0x1d0
[  293.959876]        [<ffffffffb07f00ec>] _raw_spin_lock_irqsave+0x4c/0x90
[  293.960026]        [<ffffffffb00ac070>] try_to_wake_up+0x30/0x5f0
[  293.960165]        [<ffffffffb00ac653>] wake_up_process+0x23/0x40
[  293.960300]        [<ffffffffb0f0b7a0>] rcu_spawn_gp_kthread+0xd5/0xfd
[  293.960449]        [<ffffffffb00002d3>] do_one_initcall+0xb3/0x1f0
[  293.960587]        [<ffffffffb0ef2080>] kernel_init_freeable+0x101/0x234
[  293.960736]        [<ffffffffb07e859e>] kernel_init+0xe/0xe0
[  293.960860]        [<ffffffffb07f10ff>] ret_from_fork+0x3f/0x70
[  293.960993] 
               -> #0 (rcu_node_0){-.-...}:
[  293.961081]        [<ffffffffb00d4722>] __lock_acquire+0x19d2/0x1cf0
[  293.961222]        [<ffffffffb00d53a3>] lock_acquire+0xc3/0x1d0
[  293.961351]        [<ffffffffb07f00ec>] _raw_spin_lock_irqsave+0x4c/0x90
[  293.961501]        [<ffffffffb00eb734>] rcu_report_exp_cpu_mult+0x24/0x70
[  293.961652]        [<ffffffffb00f48e9>] rcu_read_unlock_special+0x109/0x5e0
[  293.961808]        [<ffffffffb00e88f7>] __rcu_read_unlock+0x87/0x90
[  293.961947]        [<ffffffffb018059d>] perf_lock_task_context+0x19d/0x300
[  293.962101]        [<ffffffffb018b5bf>] perf_event_init_task+0x9f/0x370
[  293.962248]        [<ffffffffb007a18a>] copy_process+0x7fa/0x1e10
[  293.962385]        [<ffffffffb007b927>] _do_fork+0x97/0x680
[  293.962508]        [<ffffffffb007bfb9>] SyS_clone+0x19/0x20
[  293.962631]        [<ffffffffb07f0d97>] entry_SYSCALL_64_fastpath+0x12/0x6f
[  293.962787] 
               other info that might help us debug this:

[  293.962955] Chain exists of:
                 rcu_node_0 --> &rq->lock --> &ctx->lock

[  293.963106]  Possible unsafe locking scenario:

[  293.963233]        CPU0                    CPU1
[  293.963332]        ----                    ----
[  293.963430]   lock(&ctx->lock);
[  293.963500]                                lock(&rq->lock);
[  293.963627]                                lock(&ctx->lock);
[  293.963755]   lock(rcu_node_0);
[  293.963824] 
                *** DEADLOCK ***

[  293.963947] 2 locks held by trinity-c343/12586:
[  293.964047]  #0:  (rcu_read_lock){......}, at: [<ffffffffb01804d0>] perf_lock_task_context+0xd0/0x300
[  293.964258]  #1:  (&ctx->lock){-.....}, at: [<ffffffffb0180453>] perf_lock_task_context+0x53/0x300
[  293.972934] 
               stack backtrace:
[  293.990221] CPU: 3 PID: 12586 Comm: trinity-c343 Not tainted 4.3.0-think+ #3
[  293.999210]  ffff8804fb6824a0 ffff88044a0efa60 ffffffffb03607bc ffffffffb1779630
[  294.008115]  ffff88044a0efaa0 ffffffffb00d1453 ffff88044a0efb10 0000000000000002
[  294.017066]  ffff8804fb682468 ffff8804fb6824a0 0000000000000001 ffff8804fb681b80
[  294.026109] Call Trace:
[  294.035079]  [<ffffffffb03607bc>] dump_stack+0x4e/0x82
[  294.044014]  [<ffffffffb00d1453>] print_circular_bug+0x1e3/0x250
[  294.052982]  [<ffffffffb00d4722>] __lock_acquire+0x19d2/0x1cf0
[  294.061926]  [<ffffffffb00cedf9>] ? get_lock_stats+0x19/0x60
[  294.070878]  [<ffffffffb00d53a3>] lock_acquire+0xc3/0x1d0
[  294.079828]  [<ffffffffb00eb734>] ? rcu_report_exp_cpu_mult+0x24/0x70
[  294.088805]  [<ffffffffb07f00ec>] _raw_spin_lock_irqsave+0x4c/0x90
[  294.097771]  [<ffffffffb00eb734>] ? rcu_report_exp_cpu_mult+0x24/0x70
[  294.106807]  [<ffffffffb00eb734>] rcu_report_exp_cpu_mult+0x24/0x70
[  294.115823]  [<ffffffffb00f48e9>] rcu_read_unlock_special+0x109/0x5e0
[  294.124813]  [<ffffffffb00cedf9>] ? get_lock_stats+0x19/0x60
[  294.133801]  [<ffffffffb0180453>] ? perf_lock_task_context+0x53/0x300
[  294.142790]  [<ffffffffb00e88f7>] __rcu_read_unlock+0x87/0x90
[  294.151675]  [<ffffffffb018059d>] perf_lock_task_context+0x19d/0x300
[  294.160420]  [<ffffffffb01804d0>] ? perf_lock_task_context+0xd0/0x300
[  294.169166]  [<ffffffffb018b5bf>] perf_event_init_task+0x9f/0x370
[  294.177832]  [<ffffffffb00a64a5>] ? preempt_count_sub+0xa5/0xf0
[  294.186477]  [<ffffffffb007a18a>] copy_process+0x7fa/0x1e10
[  294.195036]  [<ffffffffb00cedf9>] ? get_lock_stats+0x19/0x60
[  294.203564]  [<ffffffffb00ced1d>] ? trace_hardirqs_off+0xd/0x10
[  294.212042]  [<ffffffffb0146101>] ? __acct_update_integrals+0x91/0xf0
[  294.220411]  [<ffffffffb00a64a5>] ? preempt_count_sub+0xa5/0xf0
[  294.228778]  [<ffffffffb007b927>] _do_fork+0x97/0x680
[  294.237038]  [<ffffffffb018fd73>] ? context_tracking_user_exit+0x13/0x20
[  294.245322]  [<ffffffffb00017ca>] ? enter_from_user_mode+0x1a/0x50
[  294.253586]  [<ffffffffb00018bc>] ? syscall_trace_enter_phase1+0xbc/0x110
[  294.261919]  [<ffffffffb0001017>] ? trace_hardirqs_on_thunk+0x17/0x19
[  294.270306]  [<ffffffffb007bfb9>] SyS_clone+0x19/0x20
[  294.278672]  [<ffffffffb07f0d97>] entry_SYSCALL_64_fastpath+0x12/0x6f