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Date:	Wed, 22 Oct 2014 11:55:11 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Cong Wang <cwang@...pensource.com>,
	Josh Boyer <jwboyer@...oraproject.org>,
	Kevin Fenzi <kevin@...ye.com>, netdev <netdev@...r.kernel.org>,
	"Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>
Subject: Re: localed stuck in recent 3.18 git in copy_net_ns?

On Wed, Oct 22, 2014 at 01:25:37PM -0500, Eric W. Biederman wrote:
> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> writes:
> 
> > On Wed, Oct 22, 2014 at 12:53:24PM -0500, Eric W. Biederman wrote:
> >> Cong Wang <cwang@...pensource.com> writes:
> >> 
> >> > (Adding Paul and Eric in Cc)
> >> >
> >> >
> >> > On Wed, Oct 22, 2014 at 10:12 AM, Josh Boyer <jwboyer@...oraproject.org> wrote:
> >> >>
> >> >> Someone else is seeing this when they try and modprobe ppp_generic:
> >> >>
> >> >> [  240.599195] INFO: task kworker/u16:5:100 blocked for more than 120 seconds.
> >> >> [  240.599338]       Not tainted 3.18.0-0.rc1.git2.1.fc22.x86_64 #1
> >> >> [  240.599446] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs"
> >> >> disables this message.
> >> >> [  240.599583] kworker/u16:5   D ffff8802202db480 12400   100      2 0x00000000
> >> >> [  240.599744] Workqueue: netns cleanup_net
> >> >> [  240.599823]  ffff8802202eb9e8 0000000000000096 ffff8802202db480
> >> >> 00000000001d5f00
> >> >> [  240.600066]  ffff8802202ebfd8 00000000001d5f00 ffff8800368c3480
> >> >> ffff8802202db480
> >> >> [  240.600228]  ffffffff81ee2690 7fffffffffffffff ffffffff81ee2698
> >> >> ffffffff81ee2690
> >> >> [  240.600386] Call Trace:
> >> >> [  240.600445]  [<ffffffff8185e239>] schedule+0x29/0x70
> >> >> [  240.600541]  [<ffffffff8186345c>] schedule_timeout+0x26c/0x410
> >> >> [  240.600651]  [<ffffffff81865ef7>] ? retint_restore_args+0x13/0x13
> >> >> [  240.600765]  [<ffffffff818644e4>] ? _raw_spin_unlock_irq+0x34/0x50
> >> >> [  240.600879]  [<ffffffff8185fc6c>] wait_for_completion+0x10c/0x150
> >> >> [  240.601025]  [<ffffffff810e53e0>] ? wake_up_state+0x20/0x20
> >> >> [  240.601133]  [<ffffffff8112a749>] _rcu_barrier+0x159/0x200
> >> >> [  240.601237]  [<ffffffff8112a845>] rcu_barrier+0x15/0x20
> >> >> [  240.601335]  [<ffffffff81718ebf>] netdev_run_todo+0x6f/0x310
> >> >> [  240.601442]  [<ffffffff8170da85>] ? rollback_registered_many+0x265/0x2e0
> >> >> [  240.601564]  [<ffffffff81725f2e>] rtnl_unlock+0xe/0x10
> >> >> [  240.601660]  [<ffffffff8170f8e6>] default_device_exit_batch+0x156/0x180
> >> >> [  240.601781]  [<ffffffff810fd8a0>] ? abort_exclusive_wait+0xb0/0xb0
> >> >> [  240.601895]  [<ffffffff81707993>] ops_exit_list.isra.1+0x53/0x60
> >> >> [  240.602028]  [<ffffffff81708540>] cleanup_net+0x100/0x1f0
> >> >> [  240.602131]  [<ffffffff810ccfa8>] process_one_work+0x218/0x850
> >> >> [  240.602241]  [<ffffffff810ccf0f>] ? process_one_work+0x17f/0x850
> >> >> [  240.602350]  [<ffffffff810cd6c7>] ? worker_thread+0xe7/0x4a0
> >> >> [  240.602454]  [<ffffffff810cd64b>] worker_thread+0x6b/0x4a0
> >> >> [  240.602555]  [<ffffffff810cd5e0>] ? process_one_work+0x850/0x850
> >> >> [  240.602665]  [<ffffffff810d399b>] kthread+0x10b/0x130
> >> >> [  240.602762]  [<ffffffff81028cc9>] ? sched_clock+0x9/0x10
> >> >> [  240.602862]  [<ffffffff810d3890>] ? kthread_create_on_node+0x250/0x250
> >> >> [  240.603004]  [<ffffffff818651fc>] ret_from_fork+0x7c/0xb0
> >> >> [  240.603106]  [<ffffffff810d3890>] ? kthread_create_on_node+0x250/0x250
> >> >> [  240.603224] 4 locks held by kworker/u16:5/100:
> >> >> [  240.603304]  #0:  ("%s""netns"){.+.+.+}, at: [<ffffffff810ccf0f>]
> >> >> process_one_work+0x17f/0x850
> >> >> [  240.603495]  #1:  (net_cleanup_work){+.+.+.}, at:
> >> >> [<ffffffff810ccf0f>] process_one_work+0x17f/0x850
> >> >> [  240.603691]  #2:  (net_mutex){+.+.+.}, at: [<ffffffff817084cc>]
> >> >> cleanup_net+0x8c/0x1f0
> >> >> [  240.603869]  #3:  (rcu_sched_state.barrier_mutex){+.+...}, at:
> >> >> [<ffffffff8112a625>] _rcu_barrier+0x35/0x200
> >> >> [  240.604211] INFO: task modprobe:1387 blocked for more than 120 seconds.
> >> >> [  240.604329]       Not tainted 3.18.0-0.rc1.git2.1.fc22.x86_64 #1
> >> >> [  240.604434] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs"
> >> >> disables this message.
> >> >> [  240.604570] modprobe        D ffff8800cb4f1a40 13112  1387   1386 0x00000080
> >> >> [  240.604719]  ffff8800cafbbbe8 0000000000000096 ffff8800cb4f1a40
> >> >> 00000000001d5f00
> >> >> [  240.604878]  ffff8800cafbbfd8 00000000001d5f00 ffff880223280000
> >> >> ffff8800cb4f1a40
> >> >> [  240.605068]  ffff8800cb4f1a40 ffffffff81f8fb48 0000000000000246
> >> >> ffff8800cb4f1a40
> >> >> [  240.605228] Call Trace:
> >> >> [  240.605283]  [<ffffffff8185e7e1>] schedule_preempt_disabled+0x31/0x80
> >> >> [  240.605400]  [<ffffffff81860033>] mutex_lock_nested+0x183/0x440
> >> >> [  240.605510]  [<ffffffff8170835f>] ? register_pernet_subsys+0x1f/0x50
> >> >> [  240.605626]  [<ffffffff8170835f>] ? register_pernet_subsys+0x1f/0x50
> >> >> [  240.605757]  [<ffffffffa0701000>] ? 0xffffffffa0701000
> >> >> [  240.605854]  [<ffffffff8170835f>] register_pernet_subsys+0x1f/0x50
> >> >> [  240.606005]  [<ffffffffa0701048>] br_init+0x48/0xd3 [bridge]
> >> >> [  240.606112]  [<ffffffff81002148>] do_one_initcall+0xd8/0x210
> >> >> [  240.606224]  [<ffffffff81153c02>] load_module+0x20c2/0x2870
> >> >> [  240.606327]  [<ffffffff8114ebe0>] ? store_uevent+0x70/0x70
> >> >> [  240.606433]  [<ffffffff8110ac26>] ? lock_release_non_nested+0x3c6/0x3d0
> >> >> [  240.606557]  [<ffffffff81154497>] SyS_init_module+0xe7/0x140
> >> >> [  240.606664]  [<ffffffff818652a9>] system_call_fastpath+0x12/0x17
> >> >> [  240.606773] 1 lock held by modprobe/1387:
> >> >> [  240.606845]  #0:  (net_mutex){+.+.+.}, at: [<ffffffff8170835f>]
> >> >> register_pernet_subsys+0x1f/0x50
> >> >> [  240.607114] INFO: task modprobe:1466 blocked for more than 120 seconds.
> >> >> [  240.607231]       Not tainted 3.18.0-0.rc1.git2.1.fc22.x86_64 #1
> >> >> [  240.607337] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs"
> >> >> disables this message.
> >> >> [  240.607473] modprobe        D ffff88020fbab480 13096  1466   1399 0x00000084
> >> >> [  240.607622]  ffff88020d1bbbe8 0000000000000096 ffff88020fbab480
> >> >> 00000000001d5f00
> >> >> [  240.607791]  ffff88020d1bbfd8 00000000001d5f00 ffffffff81e1b580
> >> >> ffff88020fbab480
> >> >> [  240.607949]  ffff88020fbab480 ffffffff81f8fb48 0000000000000246
> >> >> ffff88020fbab480
> >> >> [  240.608138] Call Trace:
> >> >> [  240.608193]  [<ffffffff8185e7e1>] schedule_preempt_disabled+0x31/0x80
> >> >> [  240.608316]  [<ffffffff81860033>] mutex_lock_nested+0x183/0x440
> >> >> [  240.608425]  [<ffffffff817083ad>] ? register_pernet_device+0x1d/0x70
> >> >> [  240.608542]  [<ffffffff817083ad>] ? register_pernet_device+0x1d/0x70
> >> >> [  240.608662]  [<ffffffffa071d000>] ? 0xffffffffa071d000
> >> >> [  240.608759]  [<ffffffff817083ad>] register_pernet_device+0x1d/0x70
> >> >> [  240.608881]  [<ffffffffa071d020>] ppp_init+0x20/0x1000 [ppp_generic]
> >> >> [  240.609021]  [<ffffffff81002148>] do_one_initcall+0xd8/0x210
> >> >> [  240.609131]  [<ffffffff81153c02>] load_module+0x20c2/0x2870
> >> >> [  240.609235]  [<ffffffff8114ebe0>] ? store_uevent+0x70/0x70
> >> >> [  240.609339]  [<ffffffff8110ac26>] ? lock_release_non_nested+0x3c6/0x3d0
> >> >> [  240.609462]  [<ffffffff81154497>] SyS_init_module+0xe7/0x140
> >> >> [  240.609568]  [<ffffffff818652a9>] system_call_fastpath+0x12/0x17
> >> >> [  240.609677] 1 lock held by modprobe/1466:
> >> >> [  240.609749]  #0:  (net_mutex){+.+.+.}, at: [<ffffffff817083ad>]
> >> >> register_pernet_device+0x1d/0x70
> >> >>
> >> >> Looks like contention on net_mutex or something, but I honestly have
> >> >> no idea yet.  I can't recreate it myself at the moment or I would
> >> >> bisect.
> >> >>
> >> >> Has nobody else run into this with the pre-3.18 kernels?  Fedora isn't
> >> >> carrying any patches in this area.
> >> 
> >> > I am not aware of any change in net/core/dev.c related here,
> >> > so I guess it's a bug in rcu_barrier().
> >> 
> >> >From the limited trace data I see in this email I have to agree.
> >> 
> >> It looks like for some reason rcu_barrier is taking forever
> >> while the rtnl_lock is held in cleanup_net.  Because the
> >> rtnl_lock is held modprobe of the ppp driver is getting stuck.
> >> 
> >> Is it possible we have an AB BA deadlock between the rtnl_lock
> >> and rcu.  With something the module loading code assumes?
> >
> > I am not aware of RCU ever acquiring rtnl_lock, not directly, anyway.
> 
> Does the module loading code do something strange with rcu?  Perhaps
> blocking an rcu grace period until the module loading completes?
> 
> If the module loading somehow blocks an rcu grace period that would
> create an AB deadlock because loading the ppp module grabs the
> rtnl_lock.  And elsewhere we have the rtnl_lock waiting for an rcu grace
> period.
> 
> I would think trying and failing to get the rtnl_lock would sleep and
> thus let any rcu grace period happen but shrug.
> 
> It looks like something is holding up the rcu grace period, and causing
> this.  Although it is possible that something is causing cleanup_net
> to run slowly and we are just seeing that slowness show up in
> rcu_barrier as that is one of the slower bits.  With a single trace I
> can't definitely same that the rcu barrier is getting stuck but it
> certainly looks that way.

Don't get me wrong -- the fact that this kthread appears to have
blocked within rcu_barrier() for 120 seconds means that something is
most definitely wrong here.  I am surprised that there are no RCU CPU
stall warnings, but perhaps the blockage is in the callback execution
rather than grace-period completion.  Or something is preventing this
kthread from starting up after the wake-up callback executes.  Or...

Is this thing reproducible?

							Thanx, Paul

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