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waitqueue_active(9) - phpMan

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WAITQUEUE_ACTIVE(9)                       Driver Basics                       WAITQUEUE_ACTIVE(9)

       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.

Kernel Hackers Manual 4.8.                 January 2017                       WAITQUEUE_ACTIVE(9)

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