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Date:	Thu, 23 Oct 2014 15:04:06 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Yanko Kaneti <yaneti@...lera.com>
Cc:	Josh Boyer <jwboyer@...oraproject.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Cong Wang <cwang@...pensource.com>,
	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 Fri, Oct 24, 2014 at 12:45:40AM +0300, Yanko Kaneti wrote:
> 
> On Thu, 2014-10-23 at 13:05 -0700, Paul E. McKenney wrote:
> > On Thu, Oct 23, 2014 at 10:51:59PM +0300, Yanko Kaneti wrote:
> > > On Thu-10/23/14-2014 08:33, Paul E. McKenney wrote:
> > > > On Thu, Oct 23, 2014 at 05:27:50AM -0700, Paul E. McKenney wrote:
> > > > > On Thu, Oct 23, 2014 at 09:09:26AM +0300, Yanko Kaneti wrote:
> > > > > > On Wed, 2014-10-22 at 16:24 -0700, Paul E. McKenney wrote:
> > > > > > > On Thu, Oct 23, 2014 at 01:40:32AM +0300, Yanko Kaneti 
> > > > > > > wrote:
> > > > > > > > On Wed-10/22/14-2014 15:33, Josh Boyer wrote:
> > > > > > > > > On Wed, Oct 22, 2014 at 2:55 PM, Paul E. McKenney
> > > > > > > > > <paulmck@...ux.vnet.ibm.com> wrote:
> > > > > > > 
> > > > > > > [ . . . ]
> > > > > > > 
> > > > > > > > > > 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?
> > > > > > > > > 
> > > > > > > > > I've added Yanko on CC, who reported the backtrace 
> > > > > > > > > above and can
> > > > > > > > > recreate it reliably.  Apparently reverting the RCU 
> > > > > > > > > merge commit
> > > > > > > > > (d6dd50e) and rebuilding the latest after that does 
> > > > > > > > > not show the
> > > > > > > > > issue.  I'll let Yanko explain more and answer any 
> > > > > > > > > questions you
> > > > > > > > > have.
> > > > > > > > 
> > > > > > > > - It is reproducible
> > > > > > > > - I've done another build here to double check and its 
> > > > > > > > definitely
> > > > > > > > the rcu merge
> > > > > > > >   that's causing it.
> > > > > > > > 
> > > > > > > > Don't think I'll be able to dig deeper, but I can do 
> > > > > > > > testing if
> > > > > > > > needed.
> > > > > > > 
> > > > > > > Please!  Does the following patch help?
> > > > > > 
> > > > > > Nope, doesn't seem to make a difference to the modprobe 
> > > > > > ppp_generic
> > > > > > test
> > > > > 
> > > > > Well, I was hoping.  I will take a closer look at the RCU 
> > > > > merge commit
> > > > > and see what suggests itself.  I am likely to ask you to 
> > > > > revert specific
> > > > > commits, if that works for you.
> > > > 
> > > > Well, rather than reverting commits, could you please try 
> > > > testing the
> > > > following commits?
> > > > 
> > > > 11ed7f934cb8 (rcu: Make nocb leader kthreads process pending 
> > > > callbacks after spawning)
> > > > 
> > > > 73a860cd58a1 (rcu: Replace flush_signals() with 
> > > > WARN_ON(signal_pending()))
> > > > 
> > > > c847f14217d5 (rcu: Avoid misordering in nocb_leader_wait())
> > > > 
> > > >         For whatever it is worth, I am guessing this one.
> > > 
> > > Indeed, c847f14217d5 it is.
> > > 
> > > Much to my embarrasment I just noticed that in addition to the
> > > rcu merge, triggering the bug "requires" my specific Fedora 
> > > rawhide network
> > > setup. Booting in single mode and modprobe ppp_generic is fine. 
> > > The bug
> > > appears when starting with my regular fedora network setup, which 
> > > in my case
> > > includes 3 ethernet adapters and a libvirt birdge+nat setup.
> > > 
> > > Hope that helps.
> > > 
> > > I am attaching the config.
> > 
> > It does help a lot, thank you!!!
> > 
> > The following patch is a bit of a shot in the dark, and assumes that
> > commit 1772947bd012 (rcu: Handle NOCB callbacks from irq-disabled 
> > idle
> > code) introduced the problem.  Does this patch fix things up?
> 
> Unfortunately not, This is linus-tip + patch

OK.  Can't have everything, I guess.

> INFO: task kworker/u16:6:96 blocked for more than 120 seconds.
>       Not tainted 3.18.0-rc1+ #4
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> kworker/u16:6   D ffff8800ca84cec0 11168    96      2 0x00000000
> Workqueue: netns cleanup_net
>  ffff8802218339e8 0000000000000096 ffff8800ca84cec0 00000000001d5f00
>  ffff880221833fd8 00000000001d5f00 ffff880223264ec0 ffff8800ca84cec0
>  ffffffff82c52040 7fffffffffffffff ffffffff81ee2658 ffffffff81ee2650
> Call Trace:
>  [<ffffffff8185b8e9>] schedule+0x29/0x70
>  [<ffffffff81860b0c>] schedule_timeout+0x26c/0x410
>  [<ffffffff81028bea>] ? native_sched_clock+0x2a/0xa0
>  [<ffffffff8110759c>] ? mark_held_locks+0x7c/0xb0
>  [<ffffffff81861b90>] ? _raw_spin_unlock_irq+0x30/0x50
>  [<ffffffff8110772d>] ? trace_hardirqs_on_caller+0x15d/0x200
>  [<ffffffff8185d31c>] wait_for_completion+0x10c/0x150
>  [<ffffffff810e4ed0>] ? wake_up_state+0x20/0x20
>  [<ffffffff8112a219>] _rcu_barrier+0x159/0x200
>  [<ffffffff8112a315>] rcu_barrier+0x15/0x20
>  [<ffffffff8171657f>] netdev_run_todo+0x6f/0x310
>  [<ffffffff8170b145>] ? rollback_registered_many+0x265/0x2e0
>  [<ffffffff817235ee>] rtnl_unlock+0xe/0x10
>  [<ffffffff8170cfa6>] default_device_exit_batch+0x156/0x180
>  [<ffffffff810fd390>] ? abort_exclusive_wait+0xb0/0xb0
>  [<ffffffff81705053>] ops_exit_list.isra.1+0x53/0x60
>  [<ffffffff81705c00>] cleanup_net+0x100/0x1f0
>  [<ffffffff810cca98>] process_one_work+0x218/0x850
>  [<ffffffff810cc9ff>] ? process_one_work+0x17f/0x850
>  [<ffffffff810cd1b7>] ? worker_thread+0xe7/0x4a0
>  [<ffffffff810cd13b>] worker_thread+0x6b/0x4a0
>  [<ffffffff810cd0d0>] ? process_one_work+0x850/0x850
>  [<ffffffff810d348b>] kthread+0x10b/0x130
>  [<ffffffff81028c69>] ? sched_clock+0x9/0x10
>  [<ffffffff810d3380>] ? kthread_create_on_node+0x250/0x250
>  [<ffffffff818628bc>] ret_from_fork+0x7c/0xb0
>  [<ffffffff810d3380>] ? kthread_create_on_node+0x250/0x250
> 4 locks held by kworker/u16:6/96:
>  #0:  ("%s""netns"){.+.+.+}, at: [<ffffffff810cc9ff>] process_one_work+0x17f/0x850
>  #1:  (net_cleanup_work){+.+.+.}, at: [<ffffffff810cc9ff>] process_one_work+0x17f/0x850
>  #2:  (net_mutex){+.+.+.}, at: [<ffffffff81705b8c>] cleanup_net+0x8c/0x1f0
>  #3:  (rcu_sched_state.barrier_mutex){+.+...}, at: [<ffffffff8112a0f5>] _rcu_barrier+0x35/0x200
> INFO: task modprobe:1045 blocked for more than 120 seconds.
>       Not tainted 3.18.0-rc1+ #4
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> modprobe        D ffff880218343480 12920  1045   1044 0x00000080
>  ffff880218353bf8 0000000000000096 ffff880218343480 00000000001d5f00
>  ffff880218353fd8 00000000001d5f00 ffffffff81e1b580 ffff880218343480
>  ffff880218343480 ffffffff81f8f748 0000000000000246 ffff880218343480
> Call Trace:
>  [<ffffffff8185be91>] schedule_preempt_disabled+0x31/0x80
>  [<ffffffff8185d6e3>] mutex_lock_nested+0x183/0x440
>  [<ffffffff81705a1f>] ? register_pernet_subsys+0x1f/0x50
>  [<ffffffff81705a1f>] ? register_pernet_subsys+0x1f/0x50
>  [<ffffffffa0673000>] ? 0xffffffffa0673000
>  [<ffffffff81705a1f>] register_pernet_subsys+0x1f/0x50
>  [<ffffffffa0673048>] br_init+0x48/0xd3 [bridge]
>  [<ffffffff81002148>] do_one_initcall+0xd8/0x210
>  [<ffffffff81153052>] load_module+0x20c2/0x2870
>  [<ffffffff8114e030>] ? store_uevent+0x70/0x70
>  [<ffffffff81278717>] ? kernel_read+0x57/0x90
>  [<ffffffff811539e6>] SyS_finit_module+0xa6/0xe0
>  [<ffffffff81862969>] system_call_fastpath+0x12/0x17
> 1 lock held by modprobe/1045:
>  #0:  (net_mutex){+.+.+.}, at: [<ffffffff81705a1f>] register_pernet_subsys+0x1f/0x50

Presumably the kworker/u16:6 completed, then modprobe hung?

If not, I have some very hard questions about why net_mutex can be
held by two tasks concurrently, given that it does not appear to be a
reader-writer lock...

Either way, my patch assumed that 39953dfd4007 (rcu: Avoid misordering in
__call_rcu_nocb_enqueue()) would work and that 1772947bd012 (rcu: Handle
NOCB callbacks from irq-disabled idle code) would fail.  Is that the case?
If not, could you please bisect the commits between 11ed7f934cb8 (rcu:
Make nocb leader kthreads process pending callbacks after spawning)
and c847f14217d5 (rcu: Avoid misordering in nocb_leader_wait())?

						Thanx, Paul

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