css_for_each_child(9)

💫 css_for_each_child(9) 💫

okay so this is a page from the Kernel API Manual — section 9, kernel API, bestie. it's called css_for_each_child and it's about looping through a css's children. yes, that kind of css. no, not the styling one 😭 this is cgroups-land.

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🎀 NAME 🎀

css_for_each_child — iterate through children of a css

✨ SYNOPSIS ✨

css_for_each_child (pos , parent );

🌸 ARGUMENTS 🌸

• pos — the css * you're using as the loop cursor. this is your little "where am i rn" pointer, she's the main character of the loop 💅
• parent — the css whose children you're walking through. the mom, basically 👑

💖 DESCRIPTION 💖

so you walk parent's children. that's the whole vibe. but — and this is the important part — it must be called under rcu_read_lock. not optional, no cap. 🔒

here's the situation with visibility, and honestly it's giving "read the fine print":

• if a subsystem synchronizes ->css_online with the start of the iteration, then a css that's finished ->css_online is guaranteed to show up in future iterations, and it will stay visible until the last reference is put. slay, stability. 🦋
• a css that hasn't finished ->css_online, or that already finished ->css_offline, may pop up during traversal. like, surprise guest energy. 🙈
• so it's each subsystem's own responsibility to synchronize against things going on/offline. nobody's gonna do it for you, bestie.

also! you're allowed to temporarily drop the RCU read lock during iteration. freedom 🕊️ but — the caller is responsible for making sure pos stays accessible until the start of the next iteration. how? for example, by bumping the css refcnt. keep your cursor alive, basically. 🫶

⚠️ quick recap of the two hard rules: call it under rcu_read_lock, and if you drop the lock mid-loop, you keep pos reachable (bump that refcnt). otherwise it's on you, fr fr.

👀 SEE ALSO 👀

kernel file ./include/linux/cgroup.h, plus the whole friend group:

• cgroup_subsys_enabled(9)
• cgroup_subsys_on_dfl(9)
• css_for_each_descendant_pre(9)
• css_for_each_descendant_post(9)
• cgroup_taskset_for_each(9)
• cgroup_taskset_for_each_leader(9)
• cgroup_css(9)
• css_is_dying(9)
• task_css_set_check(9)
• task_css_check(9)
• task_css_set(9)
• task_css(9)
• task_get_css(9)
• task_css_is_root(9)
• cgroup_is_descendant(9)
• cgroup_ancestor(9)
• cgroup_common_ancestor(9)
• task_under_cgroup_hierarchy(9)

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and that's the whole page, periodt. 💜 sign-off time: this one lives under ./include/linux in the kernel tree, dated October 2026, and it's css_for_each_child(9) — see you at the next iteration 🧚‍♀️✨