Re: Bisected: Kernel deadlock bug related to cgroups.

[Brent Lovelace]

On 08/31/2016 03:33 AM, Balbir Singh wrote:
> On 31/08/16 20:16, Oleg Nesterov wrote:
> > On 08/30, Brent Lovelace wrote:
> > > I found a kernel deadlock regression bug introduced in the 4.4 kernel.
> > ...
> > > I bisected to this commit id:
> > > ----------------------------------------------------------------------------------
> > > commit c9e75f0492b248aeaa7af8991a6fc9a21506bc96
> > > Author: Oleg Nesterov <oleg@xxxxxxxxxx>
> > > Date:   Fri Nov 27 19:57:19 2015 +0100
> > >
> > >       cgroup: pids: fix race between cgroup_post_fork() and cgroup_migrate()
> > Thanks Brent!
> >
> > > systemd         D ffff88007dfcfd10     0     1      0 0x00000000
> > >    ffff88007dfcfd10 00ff88007dfcfcd8 0000000000000001 ffff88007e296f80
> > >    ffff88007dff8000 ffff88007dfd0000 ffffffff82bae220 ffff880036758c00
> > >    fffffffffffffff2 ffff88005e327b00 ffff88007dfcfd28 ffffffff816c571f
> > > Call Trace:
> > >    [<ffffffff816c571f>] schedule+0x7a/0x8f
> > >    [<ffffffff81126ed2>] percpu_down_write+0xad/0xc4
> > >    [<ffffffff811220c8>] ? wake_up_atomic_t+0x25/0x25
> > >    [<ffffffff81169b4d>] __cgroup_procs_write+0x72/0x229
> > >    [<ffffffff8112b2eb>] ? lock_acquire+0x103/0x18f
> > so it sleeps in wait_event() waiting for active readers, and the new
> > readers will block. In particular, do_exit() will block.
> New readers/writers both, usually this is a sign that a reader is
> already holding the percpu rwsem.
>
> > > kworker/u8:2    D ffff880036befb58     0   185      2 0x00000000
> > > Workqueue: netns cleanup_net
> > >    ffff880036befb58 00ff880036befbd0 ffffffff00000002 ffff88007e316f80
> > >    ffff8800783e8000 ffff880036bf0000 ffff88005917bed0 ffff8800783e8000
> > >    ffffffff816c8953 ffff88005917bed8 ffff880036befb70 ffffffff816c571f
> > > Call Trace:
> > >    [<ffffffff816c8953>] ? usleep_range+0x3a/0x3a
> > >    [<ffffffff816c571f>] schedule+0x7a/0x8f
> > >    [<ffffffff816c8982>] schedule_timeout+0x2f/0xd8
> > >    [<ffffffff816c9204>] ? _raw_spin_unlock_irq+0x27/0x3f
> > >    [<ffffffff816c8953>] ? usleep_range+0x3a/0x3a
> > >    [<ffffffff81129ea2>] ? trace_hardirqs_on_caller+0x16f/0x18b
> > >    [<ffffffff816c5f44>] do_wait_for_common+0xf0/0x127
> > >    [<ffffffff816c5f44>] ? do_wait_for_common+0xf0/0x127
> > >    [<ffffffff811101c3>] ? wake_up_q+0x42/0x42
> > >    [<ffffffff816c5ff2>] wait_for_common+0x36/0x50
> > >    [<ffffffff816c6024>] wait_for_completion+0x18/0x1a
> > >    [<ffffffff81106b5f>] kthread_stop+0xc8/0x217
> > >    [<ffffffffa06d440c>] pg_net_exit+0xbc/0x112 [pktgen]
> > >    [<ffffffff81604fa2>] ops_exit_list+0x3d/0x4e
> > >    [<ffffffff81606123>] cleanup_net+0x19f/0x234
> > >    [<ffffffff81101aa3>] process_one_work+0x237/0x46b
> > >    [<ffffffff8110216d>] worker_thread+0x1e7/0x292
> > >    [<ffffffff81101f86>] ? rescuer_thread+0x285/0x285
> > >    [<ffffffff81106ded>] kthread+0xc4/0xcc
> > >    [<ffffffff81106d29>] ? kthread_parkme+0x1f/0x1f
> > >    [<ffffffff816c9d6f>] ret_from_fork+0x3f/0x70
> > >    [<ffffffff81106d29>] ? kthread_parkme+0x1f/0x1f
> > > 3 locks held by kworker/u8:2/185:
> > >    #0:  ("%s""netns"){.+.+.+}, at: [<ffffffff811019ad>]
> > > process_one_work+0x141/0x46b
> > >    #1:  (net_cleanup_work){+.+.+.}, at: [<ffffffff811019ad>]
> > > process_one_work+0x141/0x46b
> > >    #2:  (net_mutex){+.+.+.}, at: [<ffffffff81605ffe>] cleanup_net+0x7a/0x234
> > Note that it sleeps with net_mutex held. Probably waiting for kpktgend_*
> > below.
> >
> > > vsftpd          D ffff880054867c68     0  4352   2611 0x00000000
> > >    ffff880054867c68 00ff88005933a480 ffff880000000000 ffff88007e216f80
> > >    ffff88005933a480 ffff880054868000 0000000000000246 ffff880054867cc0
> > >    ffff88005933a480 ffffffff81cea268 ffff880054867c80 ffffffff816c571f
> > > Call Trace:
> > >    [<ffffffff816c571f>] schedule+0x7a/0x8f
> > >    [<ffffffff816c59b1>] schedule_preempt_disabled+0x10/0x19
> > >    [<ffffffff816c64c0>] mutex_lock_nested+0x1c0/0x3a0
> > >    [<ffffffff81606233>] ? copy_net_ns+0x7b/0xf8
> > >    [<ffffffff81606233>] copy_net_ns+0x7b/0xf8
> > >    [<ffffffff81606233>] ? copy_net_ns+0x7b/0xf8
> > >    [<ffffffff811073c9>] create_new_namespaces+0xfc/0x16b
> > >    [<ffffffff8110759c>] copy_namespaces+0x164/0x186
> > >    [<ffffffff810ea6b1>] copy_process+0x10d2/0x195d
> > >    [<ffffffff810eb094>] _do_fork+0x8c/0x2fb
> > >    [<ffffffff81003044>] ? lockdep_sys_exit_thunk+0x12/0x14
> > >    [<ffffffff810eb375>] SyS_clone+0x14/0x16
> > >    [<ffffffff816c99b6>] entry_SYSCALL_64_fastpath+0x16/0x76
> > > 2 locks held by vsftpd/4352:
> > >    #0:  (&cgroup_threadgroup_rwsem){++++++}, at: [<ffffffff810e9b97>]
> > > copy_process+0x5b8/0x195d
> > >    #1:  (net_mutex){+.+.+.}, at: [<ffffffff81606233>] copy_net_ns+0x7b/0xf8
> > This waits for net_mutex held by kworker/u8:2 above. And with
> > cgroup_threadgroup_rwsem acquired for reading, that is why systemd
> > above hangs.
> >
> > > kpktgend_0      D ffff88005917bce8     0  4354      2 0x00000000
> > >    ffff88005917bce8 00ffffffa06d5d06 ffff880000000000 ffff88007e216f80
> > >    ffff88007a4ec900 ffff88005917c000 ffff88007a4ec900 ffffffffa06d5d06
> > >    ffff88005917bed0 0000000000000000 ffff88005917bd00 ffffffff816c571f
> > > Call Trace:
> > >    [<ffffffffa06d5d06>] ? pg_net_init+0x346/0x346 [pktgen]
> > >    [<ffffffff816c571f>] schedule+0x7a/0x8f
> > >    [<ffffffff816c85ee>] rwsem_down_read_failed+0xdc/0xf8
> > >    [<ffffffff8136d634>] call_rwsem_down_read_failed+0x14/0x30
> > >    [<ffffffff8136d634>] ? call_rwsem_down_read_failed+0x14/0x30
> > >    [<ffffffff810f91e0>] ? exit_signals+0x17/0x103
> > >    [<ffffffff81126dc4>] ? percpu_down_read+0x4d/0x5f
> > >    [<ffffffff810f91e0>] exit_signals+0x17/0x103
> > >    [<ffffffff810ed342>] do_exit+0x105/0x9a4
> > >    [<ffffffffa06d5d06>] ? pg_net_init+0x346/0x346 [pktgen]
> > >    [<ffffffff81106df5>] kthread+0xcc/0xcc
> > >    [<ffffffff81106d29>] ? kthread_parkme+0x1f/0x1f
> > >    [<ffffffff816c9d6f>] ret_from_fork+0x3f/0x70
> > >    [<ffffffff81106d29>] ? kthread_parkme+0x1f/0x1f
> > > 1 lock held by kpktgend_0/4354:
> > >    #0:  (&cgroup_threadgroup_rwsem){++++++}, at: [<ffffffff810f91e0>]
> > it can't take cgroup_threadgroup_rwsem for reading, so it can't exit,
> > and that is why kworker/u8:2 hangs.
> >
> > > kpktgend_1      D ffff88007a4e3ce8     0  4355      2 0x00000000
> > ...
> > > kpktgend_2      D ffff8800549f7ce8     0  4356      2 0x00000000
> > ...
> > > kpktgend_3      D ffff88005e2b7ce8     0  4357      2 0x00000000
> > ...
> >
> > The same.
> >
> > Could you try the recent 568ac888215c7fb2fab "cgroup: reduce read
> > locked section of cgroup_threadgroup_rwsem during fork" patch?
> > Attached below.
> >
> >
> > With this patch copy_net_ns() should be called outside of
> > cgroup_threadgroup_rwsem, the deadlock should hopefully go away.
> >
> > Thanks,
> Yes, I would be interested in seeing if this race goes away. Thanks for
> the pointer Oleg!
>
> Balbir Singh.
Thank you so much for the help! With 568ac888215c7fb2fab patched into 
the 4.4 kernel, the deadlock issue went away.