Re: [PATCH v1] cgroup/cpuset: remove kernfs active break

[Waiman Long <llong@redhat.com>]

On 12/22/24 9:12 PM, Chen Ridong wrote:
>
> On 2024/12/20 23:13, Waiman Long wrote:
>> On 12/20/24 1:11 AM, Chen Ridong wrote:
>>> On 2024/12/20 12:16, Waiman Long wrote:
>>>> On 12/19/24 11:07 PM, chenridong wrote:
>>>>> On 2024/12/20 10:55, Waiman Long wrote:
>>>>>> On 12/19/24 8:31 PM, Chen Ridong wrote:
>>>>>>> From: Chen Ridong <chenridong@huawei.com>
>>>>>>>
>>>>>>> A warning was found:
>>>>>>>
>>>>>>> WARNING: CPU: 10 PID: 3486953 at fs/kernfs/file.c:828
>>>>>>> CPU: 10 PID: 3486953 Comm: rmdir Kdump: loaded Tainted: G
>>>>>>> RIP: 0010:kernfs_should_drain_open_files+0x1a1/0x1b0
>>>>>>> RSP: 0018:ffff8881107ef9e0 EFLAGS: 00010202
>>>>>>> RAX: 0000000080000002 RBX: ffff888154738c00 RCX: dffffc0000000000
>>>>>>> RDX: 0000000000000007 RSI: 0000000000000004 RDI: ffff888154738c04
>>>>>>> RBP: ffff888154738c04 R08: ffffffffaf27fa15 R09: ffffed102a8e7180
>>>>>>> R10: ffff888154738c07 R11: 0000000000000000 R12: ffff888154738c08
>>>>>>> R13: ffff888750f8c000 R14: ffff888750f8c0e8 R15: ffff888154738ca0
>>>>>>> FS:  00007f84cd0be740(0000) GS:ffff8887ddc00000(0000)
>>>>>>> knlGS:0000000000000000
>>>>>>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>>>>> CR2: 0000555f9fbe00c8 CR3: 0000000153eec001 CR4: 0000000000370ee0
>>>>>>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>>>>>>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>>>>>>> Call Trace:
>>>>>>>      kernfs_drain+0x15e/0x2f0
>>>>>>>      __kernfs_remove+0x165/0x300
>>>>>>>      kernfs_remove_by_name_ns+0x7b/0xc0
>>>>>>>      cgroup_rm_file+0x154/0x1c0
>>>>>>>      cgroup_addrm_files+0x1c2/0x1f0
>>>>>>>      css_clear_dir+0x77/0x110
>>>>>>>      kill_css+0x4c/0x1b0
>>>>>>>      cgroup_destroy_locked+0x194/0x380
>>>>>>>      cgroup_rmdir+0x2a/0x140
>>>>>> Were you using cgroup v1 or v2 when this warning happened?
>>>>> I was using cgroup v1.
>>>> Thanks for the confirmation.
>>>>>>> It can be explained by:
>>>>>>> rmdir                 echo 1 > cpuset.cpus
>>>>>>>                    kernfs_fop_write_iter // active=0
>>>>>>> cgroup_rm_file
>>>>>>> kernfs_remove_by_name_ns    kernfs_get_active // active=1
>>>>>>> __kernfs_remove                      // active=0x80000002
>>>>>>> kernfs_drain            cpuset_write_resmask
>>>>>>> wait_event
>>>>>>> //waiting (active == 0x80000001)
>>>>>>>                    kernfs_break_active_protection
>>>>>>>                    // active = 0x80000001
>>>>>>> // continue
>>>>>>>                    kernfs_unbreak_active_protection
>>>>>>>                    // active = 0x80000002
>>>>>>> ...
>>>>>>> kernfs_should_drain_open_files
>>>>>>> // warning occurs
>>>>>>>                    kernfs_put_active
>>>>>>>
>>>>>>> This warning is caused by 'kernfs_break_active_protection' when it is
>>>>>>> writing to cpuset.cpus, and the cgroup is removed concurrently.
>>>>>>>
>>>>>>> The commit 3a5a6d0c2b03 ("cpuset: don't nest cgroup_mutex inside
>>>>>>> get_online_cpus()") made cpuset_hotplug_workfn asynchronous, which
>>>>>>> grabs
>>>>>>> the cgroup_mutex. To avoid deadlock. the commit 76bb5ab8f6e3
>>>>>>> ("cpuset:
>>>>>>> break kernfs active protection in cpuset_write_resmask()") added
>>>>>>> 'kernfs_break_active_protection' in the cpuset_write_resmask. This
>>>>>>> could
>>>>>>> lead to this warning.
>>>>>>>
>>>>>>> After the commit 2125c0034c5d ("cgroup/cpuset: Make cpuset hotplug
>>>>>>> processing synchronous"), the cpuset_write_resmask no longer needs to
>>>>>>> wait the hotplug to finish, which means that cpuset_write_resmask
>>>>>>> won't
>>>>>>> grab the cgroup_mutex. So the deadlock doesn't exist anymore.
>>>>>>> Therefore,
>>>>>>> remove kernfs_break_active_protection operation in the
>>>>>>> 'cpuset_write_resmask'
>>>>>> The hotplug operation itself is now being done synchronously, but task
>>>>>> transfer (cgroup_transfer_tasks()) because of lacking online CPUs is
>>>>>> still being done asynchronously. So kernfs_break_active_protection()
>>>>>> will still be needed for cgroup v1.
>>>>>>
>>>>>> Cheers,
>>>>>> Longman
>>>>>>
>>>>>>
>>>>> Thank you, Longman.
>>>>> IIUC, The commit 2125c0034c5d ("cgroup/cpuset: Make cpuset hotplug
>>>>> processing synchronous") deleted the 'flush_work(&cpuset_hotplug_work)'
>>>>> in the cpuset_write_resmask. And I do not see any process within the
>>>>> cpuset_write_resmask that will grab cgroup_mutex, except for
>>>>> 'flush_work(&cpuset_hotplug_work)'.
>>>>>
>>>>> Although cgroup_transfer_tasks() is asynchronous, the
>>>>> cpuset_write_resmask will not wait any work that will grab
>>>>> cgroup_mutex.
>>>>> Consequently, the deadlock does not exist anymore.
>>>>>
>>>>> Did I miss something?
>>>> Right. The flush_work() call is still needed for a different work
>>>> function. cpuset_write_resmask() will not need to grab cgroup_mutex, but
>>>> the asynchronously executed cgroup_transfer_tasks() will. I will work on
>>>> a patch to fix that issue.
>>>>
>>>> Cheers,
>>>> Longman
>>> If flush_work() is added back, this warning still exists. Do you have a
>>> idea to fix this warning?
>> I was wrong. The flush_work() call isn't needed in this case and we
>> shouldn't need to break kernfs protection. However, your patch
>> description isn't quite right.
>>
>>> After the commit 2125c0034c5d ("cgroup/cpuset: Make cpuset hotplug
>>> processing synchronous"), the cpuset_write_resmask no longer needs to
>>> wait the hotplug to finish, which means that cpuset_write_resmask won't
>>> grab the cgroup_mutex. So the deadlock doesn't exist anymore.
>> cpuset_write_resmask() never needs to grab the cgroup_mutex. The act of
>> calling flush_work() can create a multiple processes circular locking
>> dependency that involve cgroup_mutex which can cause a deadlock. After
>> making cpuset hotplug synchronous, concurrent hotplug and cpuset
>> operations are no longer possible. However, concurrent task transfer out
>> of a previously empty CPU cpuset and adding CPU back to that cpuset is
>> possible. This will result in what the comment said "keep removing tasks
>> added
>> after execution capability is restored". That should be rare though and
>> we should probably add a check in cgroup_transfer_tasks() to detect such
>> a case and break out of it.
>>
>> Cheers,
>> Longman
> Hi, Longman, sorry the confused message. Do you mean this patch is
> acceptable if I update the message?
Sorry for the late reply. Yes, the patch is acceptable, but the patch 
description isn't quite right. Please sent out a v2.
>
> I don't think we need to add a check in the cgroup_transfer_tasks
> function. Because no process(except for writing cpuset.cpus, which has
> been reoved) will need 'kn->active' to involve cgroup_transfer_tasks now.

I agree that we don't need to add a check in cgroup_transfer_tasks().

Cheers,
Longman