Function sync_fsync

freebsd/kern/vfs_subr.c:5040–5083

* Do a lazy sync of the filesystem. */

 * Do a lazy sync of the filesystem.
 */
static int
sync_fsync(struct vop_fsync_args *ap)
{
	struct vnode *syncvp = ap->a_vp;
	struct mount *mp = syncvp->v_mount;
	int error, save;
	struct bufobj *bo;

	/*
	 * We only need to do something if this is a lazy evaluation.
	 */
	if (ap->a_waitfor != MNT_LAZY)
		return (0);

	/*
	 * Move ourselves to the back of the sync list.
	 */
	bo = &syncvp->v_bufobj;
	BO_LOCK(bo);
	vn_syncer_add_to_worklist(bo, syncdelay);
	BO_UNLOCK(bo);

	/*
	 * Walk the list of vnodes pushing all that are dirty and
	 * not already on the sync list.
	 */
	if (vfs_busy(mp, MBF_NOWAIT) != 0)
		return (0);
	if (vn_start_write(NULL, &mp, V_NOWAIT) != 0) {
		vfs_unbusy(mp);
		return (0);
	}
	save = curthread_pflags_set(TDP_SYNCIO);
	/*
	 * The filesystem at hand may be idle with free vnodes stored in the
	 * batch.  Return them instead of letting them stay there indefinitely.
	 */
	vfs_periodic(mp, MNT_NOWAIT);
	error = VFS_SYNC(mp, MNT_LAZY);
	curthread_pflags_restore(save);
	vn_finished_write(mp);
	vfs_unbusy(mp);
	return (error);
}

/*
 * The syncer vnode is no referenced.