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

[Waiman Long <llong@redhat.com>]

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