Man page 9 : __wake_up_sync_key

__wake_up_sync_key - wake up threads blocked on a waitqueue.

void __wake_up_sync_key(wait_queue_head_t *  q , unsigned int  mode , int  nr_exclusive , void *  key );

q
    the waitqueue

mode
    which threads

nr_exclusive
    how many wake-one or wake-many threads to wake up

key
    opaque value to be passed to wakeup targets

The sync wakeup differs that the waker knows that it will schedule away soon, so while the target thread will be woken up, it will not be migrated to another CPU - ie. the two threads are AqsynchronizedAq with each other. This can prevent needless bouncing between CPUs.

On UP it can prevent extra preemption.

It may be assumed that this function implies a write memory barrier before changing the task state if and only if any tasks are woken up.