waitqueue_active(9)

💖 NAME 💖

okay so this is waitqueue_active(9), straight out of the Kernel API Manual, and it’s here to locklessly test for waiters on the queue. That’s the whole name vibe, bestie. ✨

---

✨ SYNOPSIS ✨

int waitqueue_active (struct wait_queue_head *wq_head );

---

🦋 ARGUMENTS 🦋

• wq_head — the waitqueue to test for waiters. That’s it, that’s the argument. 🫶

---

💅 DESCRIPTION 💅

It returns true if the wait list is not empty. That’s the behavior, no cap. 👑

---

⚠️ NOTE ⚠️

this function is lockless and requires care, incorrect usage _will_ lead to sporadic and non-obvious failure. 💀

Use either while holding wait_queue_head::lock or when used for wakeups with an extra smp_mb like:

            CPU0 - waker                    CPU1 - waiter

                                            for (;;) {
            cond = true;                     prepare_to_wait(wq_head, wait, state);
            smp_mb;                         // smp_mb from set_current_state
            if (waitqueue_active(wq_head))         if (cond)
              wake_up(wq_head);                      break;
                                              schedule;
                                            }
                                            finish_wait(wq_head, wait);

Because without the explicit smp_mb it's possible for the waitqueue_active load to get hoisted over the cond store such that we'll observe an empty wait list while the waiter might not observe cond.

Also note that this 'optimization' trades a spin_lock for an smp_mb, which (when the lock is uncontended) are of roughly equal cost.

---

🌈 SEE ALSO 🌈

Kernel file ./include/linux/wait.h

• wq_has_single_sleeper(9)
• wq_has_sleeper(9)
• wake_up_pollfree(9)
• wait_event(9)
• wait_event_freezable(9)
• wait_event_timeout(9)
• wait_event_cmd(9)
• wait_event_interruptible(9)
• wait_event_interruptible_timeout(9)
• wait_event_hrtimeout(9)
• wait_event_interruptible_hrtimeout(9)
• wait_event_idle(9)
• wait_event_idle_exclusive(9)
• wait_event_idle_timeout(9)
• wait_event_idle_exclusive_timeout(9)
• wait_event_interruptible_locked(9)
• wait_event_interruptible_locked_irq(9)
• wait_event_interruptible_exclusive_locked(9)
• wait_event_interruptible_exclusive_locked_irq(9)
• wait_event_killable(9)
• wait_event_state(9)
• wait_event_killable_timeout(9)
• wait_event_lock_irq_cmd(9)
• wait_event_lock_irq(9)
• wait_event_interruptible_lock_irq_cmd(9)
• wait_event_interruptible_lock_irq(9)
• wait_event_interruptible_lock_irq_timeout(9)

okay bestie, that’s the whole page — source/version: ./include/linux, date: October 2026, page: waitqueue_active(9) — byeee, go check your wait list 💖👋