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

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Message-ID: <d3ebff6a-9866-40e2-a1ff-07bd77d20187@huaweicloud.com>
Date: Fri, 20 Dec 2024 14:11:39 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: Waiman Long <llong@...hat.com>, chenridong <chenridong@...wei.com>,
 tj@...nel.org, hannes@...xchg.org, mkoutny@...e.com, roman.gushchin@...ux.dev
Cc: cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
 bpf@...r.kernel.org, wangweiyang2@...wei.com
Subject: Re: [PATCH v1] cgroup/cpuset: remove kernfs active break



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@...wei.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?

Best regards
Ridong