Re: [PATCH 1/6] sched/wait: Add swq_has_sleepers()

[Paolo Bonzini]

On 11/10/2017 10:27, Peter Zijlstra wrote:
> On Tue, Sep 05, 2017 at 12:00:17PM -0700, Davidlohr Bueso wrote:
>> Which is the equivalent of what we have in regular waitqueues.
>> I'm not crazy about the name, but this also helps us get both
>> apis closer -- which iirc comes originally from the -net folks.
> 
> I'm confused. I don't see anything like this in the original waitqueue.
> 

/**
 * wq_has_sleeper - check if there are any waiting processes
 * @wq_head: wait queue head
 *
 * Returns true if wq_head has waiting processes
 *
 * Please refer to the comment for waitqueue_active.
 */
static inline bool wq_has_sleeper(struct wait_queue_head *wq_head)
{
        /*
         * We need to be sure we are in sync with the
         * add_wait_queue modifications to the wait queue.
         *
         * This memory barrier should be paired with one on the
         * waiting side.
         */
        smp_mb();
        return waitqueue_active(wq_head);
}

Paolo