Re: Bisected: Kernel deadlock bug related to cgroups.

[Balbir Singh]

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.