check_flush_dependency(9)

💅 check_flush_dependency(9) — the kernel API manual, bestie edition

okay so this is section 9 of the kernel API manual, and today's main character is check_flush_dependency ✨

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🌸 NAME 🌸

check_flush_dependency — literally just checking for flush dependency sanity 🫶

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✨ SYNOPSIS ✨

void check_flush_dependency (struct workqueue_struct *target_wq , struct
work_struct *target_work , bool from_cancel );

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🎀 ARGUMENTS 🎀

• target_wq — the workqueue that's being flushed 💅
• target_work — the work item that's being flushed (it's NULL when it's a whole-workqueue flush, no cap)
• from_cancel — are we being called from the work cancel path? like, yes or no, tell us 🙈

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🦋 DESCRIPTION 🦋

so here's the vibe: current is trying to flush the whole target_wq, or target_work on it. periodt.

now, if this isn't the cancel path — and just so we're clear, the cancel path implies the work being flushed is either already running or will not be at all — then we gotta check stuff 👑

we check whether target_wq doesn't have WQ_MEM_RECLAIM, and we verify that current is not reclaiming memory and is not running on a workqueue that doesn't have WQ_MEM_RECLAIM — because that can break the forward-progress guarantee, which leads to a deadlock 💀 and nobody wants that.

lowkey important: that's a deadlock. the whole system just sits there. not the look. 😭

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💜 SEE ALSO 💜

the kernel file ./kernel/workqueue.c is the source of truth here, and it comes with a whole friend group — for_each_pool(9), for_each_pool_worker(9), for_each_pwq(9), worker_pool_assign_id(9), unbound_effective_cpumask(9), get_work_pool(9), worker_set_flags(9), worker_clr_flags(9), worker_enter_idle(9), worker_leave_idle(9), find_worker_executing_work(9), move_linked_works(9), assign_work(9), kick_pool_pick(9), kick_pool(9), wq_worker_running(9), wq_worker_sleeping(9), wq_worker_tick(9), wq_worker_last_func(9), wq_node_nr_active(9), wq_update_node_max_active(9), get_pwq(9), put_pwq(9), put_pwq_unlocked(9), pwq_tryinc_nr_active(9), pwq_activate_first_inactive(9), unplug_oldest_pwq(9), node_activate_pending_pwq(9), pwq_dec_nr_active(9), pwq_dec_nr_in_flight(9), try_to_grab_pending(9), work_grab_pending(9), insert_work(9), queue_work_on(9), select_numa_node_cpu(9), queue_work_node(9), queue_delayed_work_on(9), mod_delayed_work_on(9), queue_rcu_work(9), worker_attach_to_pool(9), worker_detach_from_pool(9), create_worker(9), set_worker_dying(9), idle_worker_timeout(9), idle_cull_fn(9), maybe_create_worker(9), manage_workers(9), process_one_work(9), process_scheduled_works(9), worker_thread(9), rescuer_thread(9), insert_wq_barrier(9), flush_workqueue_prep_pwqs(9), __flush_workqueue(9), drain_workqueue(9), flush_work(9), flush_delayed_work(9), flush_rcu_work(9), cancel_work_sync(9), cancel_delayed_work(9), cancel_delayed_work_sync(9), disable_work(9), disable_work_sync(9), enable_work(9), disable_delayed_work(9), disable_delayed_work_sync(9), enable_delayed_work(9), schedule_on_each_cpu(9), execute_in_process_context(9), free_workqueue_attrs(9), alloc_workqueue_attrs(9), init_worker_pool(9), put_unbound_pool(9), get_unbound_pool(9), wq_calc_pod_cpumask(9), apply_workqueue_attrs(9), unbound_wq_update_pwq(9), wq_adjust_max_active(9), destroy_workqueue(9), workqueue_set_max_active(9), workqueue_set_min_active(9), current_work(9), current_is_workqueue_rescuer(9), current_is_workqueue_mem_reclaim(9), workqueue_congested(9), work_busy(9), set_worker_desc(9), print_worker_info(9), show_one_workqueue(9), show_one_worker_pool(9), show_all_workqueues(9), show_freezable_workqueues(9), rebind_workers(9), restore_unbound_workers_cpumask(9), work_on_cpu_key(9), freeze_workqueues_begin(9), freeze_workqueues_busy(9), thaw_workqueues(9), workqueue_unbound_housekeeping_update(9), workqueue_set_unbound_cpumask(9), workqueue_sysfs_register(9), workqueue_sysfs_unregister(9), workqueue_init_early(9), workqueue_init(9), llc_count_cores(9), llc_populate_cpu_shard_id(9), precompute_cache_shard_ids(9), workqueue_init_topology(9) — yeah, that's the whole group chat, read every single one or you're not in the gc anymore 🧚

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and that's the page!! it's from ./kernel, dated October 2026 ⭐ slay, go read your workqueue internals bestie 💖🌸