In the Linux kernel, the following vulnerability has been resolved:

cgroup: split cgroup_destroy_wq into 3 workqueues

A hung task can occur during [1] LTP cgroup testing when repeatedly
mounting/unmounting perf_event and net_prio controllers with
systemd.unified_cgroup_hierarchy=1. The hang manifests in
cgroup_lock_and_drain_offline() during root destruction.

Related case:
cgroup_fj_function_perf_event cgroup_fj_function.sh perf_event
cgroup_fj_function_net_prio cgroup_fj_function.sh net_prio

Call Trace:
	cgroup_lock_and_drain_offline+0x14c/0x1e8
	cgroup_destroy_root+0x3c/0x2c0
	css_free_rwork_fn+0x248/0x338
	process_one_work+0x16c/0x3b8
	worker_thread+0x22c/0x3b0
	kthread+0xec/0x100
	ret_from_fork+0x10/0x20

Root Cause:

CPU0                            CPU1
mount perf_event                umount net_prio
cgroup1_get_tree                cgroup_kill_sb
rebind_subsystems               // root destruction enqueues
				// cgroup_destroy_wq
// kill all perf_event css
                                // one perf_event css A is dying
                                // css A offline enqueues cgroup_destroy_wq
                                // root destruction will be executed first
                                css_free_rwork_fn
                                cgroup_destroy_root
                                cgroup_lock_and_drain_offline
                                // some perf descendants are dying
                                // cgroup_destroy_wq max_active = 1
                                // waiting for css A to die

Problem scenario:
1. CPU0 mounts perf_event (rebind_subsystems)
2. CPU1 unmounts net_prio (cgroup_kill_sb), queuing root destruction work
3. A dying perf_event CSS gets queued for offline after root destruction
4. Root destruction waits for offline completion, but offline work is
   blocked behind root destruction in cgroup_destroy_wq (max_active=1)

Solution:
Split cgroup_destroy_wq into three dedicated workqueues:
cgroup_offline_wq – Handles CSS offline operations
cgroup_release_wq – Manages resource release
cgroup_free_wq – Performs final memory deallocation

This separation eliminates blocking in the CSS free path while waiting for
offline operations to complete.

[1] https://github.com/linux-test-project/ltp/blob/master/runtest/controllers

https://git.kernel.org/stable/c/05e0b03447cf215ec384210441b34b7a3b16e8b0

https://git.kernel.org/stable/c/4a1e3ec28e8062cd9f339aa6a942df9c5bcb6811

https://git.kernel.org/stable/c/79f919a89c9d06816dbdbbd168fa41d27411a7f9

https://git.kernel.org/stable/c/993049c9b1355c78918344a6403427d53f9ee700

https://git.kernel.org/stable/c/a0c896bda7077aa5005473e2c5b3c27173313b4c

https://git.kernel.org/stable/c/cabadd7fd15f97090f752fd22dd7f876a0dc3dc4

https://git.kernel.org/stable/c/ded4d207a3209a834b6831ceec7f39b934c74802

https://git.kernel.org/stable/c/f2795d1b92506e3adf52a298f7181032a1525e04