void blk_start_plug(struct blk_plug *plug)
{
	struct task_struct *tsk = current;

	/*
	 * If this is a nested plug, don't actually assign it.
	 */
	if (tsk->plug)
		return;

	INIT_LIST_HEAD(&plug->list);
	INIT_LIST_HEAD(&plug->mq_list);
	INIT_LIST_HEAD(&plug->cb_list);
	/*
	 * Store ordering should not be needed here, since a potential
	 * preempt will imply a full memory barrier
	 */
	tsk->plug = plug;
}

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.