waitqueue_active — - locklessly test for waiters on the queue


int waitqueue_active (wait_queue_head_t * q);



the waitqueue to test for waiters


returns true if the wait list is not empty


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

Use either while holding wait_queue_head_t

:lock or when used for wakeups with an extra smp_mb like:

CPU0 - waker CPU1 - waiter

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