static boolean_t
arc_adjust_cb_check(void *arg, zthr_t *zthr)
{
        /*
         * We have to rely on arc_get_data_impl() to tell us when to adjust,
         * rather than checking if we are overflowing here, so that we are
         * sure to not leave arc_get_data_impl() waiting on
         * arc_adjust_waiters_cv.  If we have become "not overflowing" since
         * arc_get_data_impl() checked, we need to wake it up.  We could
         * broadcast the CV here, but arc_get_data_impl() may have not yet
         * gone to sleep.  We would need to use a mutex to ensure that this
         * function doesn't broadcast until arc_get_data_impl() has gone to
         * sleep (e.g. the arc_adjust_lock).  However, the lock ordering of
         * such a lock would necessarily be incorrect with respect to the
         * zthr_lock, which is held before this function is called, and is
         * held by arc_get_data_impl() when it calls zthr_wakeup().
         */
        return (arc_adjust_needed);
}
static void
arc_reduce_target_size(int64_t to_free)
{
...
        if (asize > arc_c) {
                DTRACE_PROBE2(arc__shrink_adjust, uint64_t, asize,
                        uint64_t, arc_c);
                /* See comment in arc_adjust_cb_check() on why lock+flag */
                mutex_enter(&arc_adjust_lock);
                arc_adjust_needed = B_TRUE;
                mutex_exit(&arc_adjust_lock);
                zthr_wakeup(arc_adjust_zthr);
        }
}