[GFS2] Revise readpage locking

 * @file: The file to read

 * @page: The page of the file

 *

 * This deals with the locking required. We use a trylock in order to

 * avoid the page lock / glock ordering problems returning AOP_TRUNCATED_PAGE

 * in the event that we are unable to get the lock.

 * This deals with the locking required. We have to unlock and

 * relock the page in order to get the locking in the right

 * order.

 */

static int gfs2_readpage(struct file *file, struct page *page)

{

	struct gfs2_inode *ip = GFS2_I(page->mapping->host);

	struct address_space *mapping = page->mapping;

	struct gfs2_inode *ip = GFS2_I(mapping->host);

	struct gfs2_holder gh;

	int error;

	gfs2_holder_init(ip->i_gl, LM_ST_SHARED, GL_ATIME|LM_FLAG_TRY_1CB, &gh);

	error = gfs2_glock_nq_atime(&gh);

	if (unlikely(error)) {

	unlock_page(page);

	gfs2_holder_init(ip->i_gl, LM_ST_SHARED, GL_ATIME, &gh);

	error = gfs2_glock_nq_atime(&gh);

	if (unlikely(error))

		goto out;

	}

	error = AOP_TRUNCATED_PAGE;

	lock_page(page);

	if (page->mapping == mapping && !PageUptodate(page))

		error = __gfs2_readpage(file, page);

	else

		unlock_page(page);

	gfs2_glock_dq(&gh);

out:

	gfs2_holder_uninit(&gh);

	if (error == GLR_TRYFAILED) {

		yield();

		return AOP_TRUNCATED_PAGE;

	}

	if (error && error != AOP_TRUNCATED_PAGE)

		lock_page(page);

	return error;

}