css_next_descendant_post(9)

🌸 css_next_descendant_post(9) — Kernel API Manual, but make it make sense 🌸

okay bestie, welcome to the Kernel API Manual page for css_next_descendant_post(9) — the traversal helper that finds the next descendant for a post-order walk. It's giving cgroup tree energy, and we're keeping every detail, fr fr. 💖

✨ NAME ✨

css_next_descendant_post — find the next descendant for post-order walk

🦋 SYNOPSIS 🦋

struct cgroup_subsys_state * css_next_descendant_post (struct
cgroup_subsys_state *pos , struct cgroup_subsys_state *root );

🎀 ARGUMENTS 🎀

• pos — the current position, and you pass NULL to initiate traversal. iconic starting move.
• root — the css whose descendants to walk. That's the main character tree, periodt.

💅 DESCRIPTION 💅

This one is meant to be used by css_for_each_descendant_post, okay? Its whole job: find the next descendant to visit for post-order traversal of root's descendants. And yes, root is included in the iteration and is the last node to be visited — no cap. 👑

While this function requires cgroup_mutex or RCU read locking, it doesn't require the whole traversal to be contained in a single critical section. Also, it isn't necessary to hold onto a reference to pos. This function will return the correct next descendant as long as both pos and cgroup are accessible and pos is a descendant of cgroup. that's the vibe check ✨

If a subsystem synchronizes ->css_online and the start of iteration, a css which finished ->css_online is guaranteed to be visible in the future iterations and will stay visible until the last reference is put. A css which hasn't finished ->css_online or already finished ->css_offline may show up during traversal. It's each subsystem's responsibility to synchronize against on/offlining. pls handle your own on/offlining business, bestie 🫶

lowkey important: the locking caveat is real — keep cgroup_mutex or RCU read locking in mind, and don't assume the whole traversal sits inside one critical section. 💀

🌈 SEE ALSO 🌈

Kernel file ./kernel/cgroup/cgroup.c — and then the whole friend group:

• cgroup_ssid_enabled(9), cgroup_on_dfl(9), cgroup_e_css_by_mask(9), cgroup_e_css(9), cgroup_get_e_css(9), __cgroup_task_count(9), cgroup_task_count(9)
• for_each_css(9), do_each_subsys_mask(9), css_set_populated(9), css_update_populated(9), css_set_update_populated(9), css_set_move_task(9), compare_css_sets(9), find_existing_css_set(9)
• allocate_cgrp_cset_links(9), link_css_set(9), find_css_set(9), cgroup_file_mode(9), cgroup_calc_subtree_ss_mask(9), cgroup_kn_unlock(9), cgroup_kn_lock_live(9), css_clear_dir(9)
• css_populate_dir(9), cgroup_attach_lock(9), cgroup_attach_unlock(9), cgroup_migrate_add_task(9), cgroup_taskset_first(9), cgroup_taskset_next(9), cgroup_migrate_execute(9)
• cgroup_migrate_vet_dst(9), cgroup_migrate_finish(9), cgroup_migrate_add_src(9), cgroup_migrate_prepare_dst(9), cgroup_migrate(9), cgroup_attach_task(9), cgroup_update_dfl_csses(9)
• cgroup_lock_and_drain_offline(9), cgroup_save_control(9), cgroup_propagate_control(9), cgroup_restore_control(9), cgroup_apply_control_enable(9), cgroup_apply_control_disable(9)
• cgroup_apply_control(9), cgroup_finalize_control(9), cgroup_enable_threaded(9), cgroup_tryget_css(9), cgroup_addrm_files(9), cgroup_rm_cftypes(9), cgroup_add_cftypes(9), cgroup_add_dfl_cftypes(9)
• cgroup_add_legacy_cftypes(9), cgroup_file_notify(9), cgroup_file_show(9), css_next_child(9), css_next_descendant_pre(9), css_rightmost_descendant(9), css_has_online_children(9)
• css_task_iter_advance_css_set(9), css_task_iter_start(9), css_task_iter_next(9), css_task_iter_end(9), css_create(9), kill_css_sync(9), kill_css_finish(9)
• cgroup_destroy_locked(9), cgroup_init_early(9), cgroup_init(9), cgroup_fork(9), cgroup_v1v2_get_from_file(9), cgroup_get_from_file(9), cgroup_css_set_fork(9), cgroup_css_set_put_fork(9)
• cgroup_can_fork(9), cgroup_cancel_fork(9), cgroup_post_fork(9), cgroup_task_exit(9), css_tryget_online_from_dir(9), css_from_id(9), cgroup_get_from_path(9)
• cgroup_v1v2_get_from_fd(9), cgroup_get_from_fd(9), cgroup_parse_float(9)

okay that's the whole list, no skipping, no cap. 🍓

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and that's the page, bestie. source/version: ./kernel/cgroup, date: October 2026 — mwah, go walk that tree post-order and slay 💋✨