waitqueue_active — - locklessly test for waiters on the queue
int waitqueue_active ( | wait_queue_head_t * q); |
this function is lockless and requires care, incorrect usage _will_ lead to sporadic and non-obvious failure.
: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.