From Eric Anholt:

Return EBUSY when attempting to addmap a DRM_SHM area with a lock in it if
dev->lock.hw_lock is already set. This fixes the case of two X Servers running
on the same head on different VTs with interface 1.1, by making the 2nd head
fail to inizialize like before.

drivers/char/drm/drm_bufs.h

@@ -160,6 +160,12 @@ int DRM(addmap)( struct inode *inode, struct file *filp,
 		}
 		map->offset = (unsigned long)map->handle;
 		if ( map->flags & _DRM_CONTAINS_LOCK ) {
+			/* Prevent a 2nd X Server from creating a 2nd lock */
+			if (dev->lock.hw_lock != NULL) {
+				vfree( map->handle );
+				DRM(free)( map, sizeof(*map), DRM_MEM_MAPS );
+				return -EBUSY;
+			}
 			dev->sigdata.lock =
 			dev->lock.hw_lock = map->handle; /* Pointer to lock */
 		}