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

[chenridong]

On 2024/12/20 10:55, Waiman Long wrote:
> On 12/19/24 8:31 PM, Chen Ridong wrote:
>> From: Chen Ridong <chenridong@xxxxxxxxxx>
>>
>> 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.

>>
>> 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?

Best regards
Ridong