BUG: tracer_alloc_buffers returned with preemption imbalance

[Fengguang Wu]

Hi Paul,

I got some boot hangs in the below branches and linux-next:

        rcu/dev.2012.06.21c-4f4a59a
        rcu/fixes.2012.06.20a-b43145c
        rcu/rcu.next-3f26dd3

They all start with an error

[    0.131005] initcall tracer_alloc_buffers+0x0/0x22e returned with preemption imbalance

Then followed by lots of scheduling while atomic bugs:

[    0.143206] Brought up 2 CPUs
[    0.144996] ----------------
[    0.145989] | NMI testsuite:
[    0.147988] --------------------
[    0.149238]   remote IPI:
[    0.149239] BUG: scheduling while atomic: swapper/0/1/0x00000002
[    0.151082] no locks held by swapper/0/1.
[    0.152991] Modules linked in:
[    0.153992] Pid: 1, comm: swapper/0 Not tainted 3.5.0-rc3+ #24
[    0.155990] Call Trace:
[    0.157056]  [<ffffffff81a21ee1>] __schedule_bug+0x67/0x75
[    0.158999]  [<ffffffff81a3717c>] __schedule+0xb0/0x8b0
[    0.160995]  [<ffffffff810b8fe2>] ? save_trace+0x3d/0xae
[    0.162993]  [<ffffffff8105ad19>] ? pvclock_clocksource_read+0x4d/0xc3
[    0.164992]  [<ffffffff810bbdff>] ? mark_lock+0x23c/0x502
[    0.166993]  [<ffffffff8105a10b>] ? kvm_clock_read+0x2e/0x36
[    0.168991]  [<ffffffff81a37c61>] schedule+0x65/0x67
[    0.170992]  [<ffffffff81a35542>] schedule_timeout+0x2e/0x22a
[    0.172991]  [<ffffffff810b8c1c>] ? trace_hardirqs_off+0xd/0xf
[    0.174992]  [<ffffffff8109c890>] ? local_clock+0x41/0x5a
[    0.176995]  [<ffffffff81a392f0>] ? _raw_spin_unlock_irq+0x2e/0x48
[    0.178990]  [<ffffffff810bd647>] ? trace_hardirqs_on_caller+0x125/0x181
[    0.180995]  [<ffffffff810bd6b0>] ? trace_hardirqs_on+0xd/0xf
[    0.182989]  [<ffffffff81a37b06>] wait_for_common+0x10a/0x140
[    0.184989]  [<ffffffff8109a2f2>] ? try_to_wake_up+0x2cb/0x2cb
[    0.186990]  [<ffffffff810ea4c6>] ? call_rcu_bh+0x19/0x19
[    0.188988]  [<ffffffff81a37bfa>] wait_for_completion+0x1d/0x1f
[    0.190989]  [<ffffffff8108752d>] wait_rcu_gp+0x4d/0x54
[    0.192988]  [<ffffffff81087534>] ? wait_rcu_gp+0x54/0x54
[    0.194987]  [<ffffffff81a37a40>] ? wait_for_common+0x44/0x140
[    0.196987]  [<ffffffff810e8713>] synchronize_sched+0x50/0x52
[    0.198987]  [<ffffffff8103b272>] unregister_nmi_handler+0xc6/0xd5
[    0.200989]  [<ffffffff8142fa79>] ? delay_tsc+0x49/0xa5
[    0.202988]  [<ffffffff82259db7>] test_nmi_ipi.constprop.2+0x72/0x85
[    0.204986]  [<ffffffff82259dca>] ? test_nmi_ipi.constprop.2+0x85/0x85
[    0.206986]  [<ffffffff82259e27>] remote_ipi+0x5d/0x62
[    0.208985]  [<ffffffff82259ce2>] dotest.constprop.1+0x6/0x69
[    0.210985]  [<ffffffff82259ede>] nmi_selftest+0x60/0x16d
[    0.212986]  [<ffffffff822558da>] native_smp_cpus_done+0x1c/0x111
[    0.214989]  [<ffffffff8226198b>] smp_init+0xe2/0xeb
[    0.216986]  [<ffffffff82248c0f>] kernel_init+0xa9/0x1c0
[    0.218984]  [<ffffffff810bd538>] ? trace_hardirqs_on_caller+0x16/0x181
[    0.220989]  [<ffffffff81a417b4>] kernel_thread_helper+0x4/0x10
[    0.222989]  [<ffffffff81a396f0>] ? retint_restore_args+0x13/0x13
[    0.224990]  [<ffffffff82248b66>] ? start_kernel+0x3a3/0x3a3
[    0.226983]  [<ffffffff81a417b0>] ? gs_change+0x13/0x13
[    0.231002]   ok  |
[    0.231998]    local IPI:
[    0.231998] BUG: scheduling while atomic: swapper/0/1/0x00000003
...

The first bad commit is

commit b43145c912b7dabd190ec45cc31c2b96d64715ee
Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Date:   Tue Jun 19 11:58:27 2012 -0700

    rcu: Disable preemption in rcu_blocking_is_gp()
    
    It is time to optimize CONFIG_TREE_PREEMPT_RCU's synchronize_rcu()
    for uniprocessor optimization, which means that rcu_blocking_is_gp()
    can no longer rely on RCU read-side critical sections having disabled
    preemption.  This commit therefore disables preemption across
    rcu_blocking_is_gp()'s scan of the cpu_online_mask.
    
    Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index ce175b6..c0cc41f 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
...
 static inline int rcu_blocking_is_gp(void)
 {
        might_sleep();  /* Check for RCU read-side critical section. */
+       preempt_disable();
        return num_online_cpus() <= 1;
+       preempt_enable();
 }

Thanks,
Fengguang
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