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

[Waiman Long <llong@redhat.com>]

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?
>
> 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