显示在wordpress上按标记过滤的帖子？

I want to query all posts that contain an specific tag, so, lookin on the codex ref, it says something like this should work:

$args = array( 'tag' => 'my-tag' );
$wp_query->query($args);
while ( $wp_query->have_posts() ) :
    $wp_query->the_post();
 endwhile;

But it always returns false...

I was reading is not possible to do that, and I have to use the has_tag while i do a while over all the posts, is it true??

Any idea how to do this?

Thanks,

Very simple, just requires reading: http://codex.wordpress.org/Class_Reference/WP_Query

<?php $your_query = new WP_query(' tag_id=###' );

    if ( $your_query->have_posts() ) : ?>

    // This begins the loop to post the posts with your tag
    <?php while ( $your_query->have_posts() ) : $your_query->the_post(); ?>
        <h2><?php the_title(); ?></h2>
    <?php endwhile; ?>
    // Closes loop

    <?php wp_reset_postdata(); ?>

<?php else : ?>
    // Loads error if no posts exists
    <p><?php _e( 'Sorry, no posts matched your criteria.' ); ?></p>
<?php endif; ?>