Man page 9 : complete

complete - signals a single thread waiting on this completion

void complete(struct completion *  x );

x
    holds the state of this particular completion

This will wake up a single thread waiting on this completion. Threads will be awakened in the same order in which they were queued.

See also complete_all, wait_for_completion and related routines.

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.