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@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.
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
