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SubjectBUG: tracer_alloc_buffers returned with preemption imbalance
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@linux.vnet.ibm.com>
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@linux.vnet.ibm.com>

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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