int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags)
{
	const bool preempt = flags & BLK_MQ_REQ_PREEMPT;

	while (true) {
		bool success = false;

		rcu_read_lock();
		if (percpu_ref_tryget_live(&q->q_usage_counter)) {
			/*
			 * The code that sets the PREEMPT_ONLY flag is
			 * responsible for ensuring that that flag is globally
			 * visible before the queue is unfrozen.
			 */
			if (preempt || !blk_queue_preempt_only(q)) {
				success = true;
			} else {
				percpu_ref_put(&q->q_usage_counter);
			}
		}
		rcu_read_unlock();

		if (success)
			return 0;

		if (flags & BLK_MQ_REQ_NOWAIT)
			return -EBUSY;

		/*
		 * read pair of barrier in blk_freeze_queue_start(),
		 * we need to order reading __PERCPU_REF_DEAD flag of
		 * .q_usage_counter and reading .mq_freeze_depth or
		 * queue dying flag, otherwise the following wait may
		 * never return if the two reads are reordered.
		 */
		smp_rmb();

		wait_event(q->mq_freeze_wq,
			   (atomic_read(&q->mq_freeze_depth) == 0 &&
			    (preempt || !blk_queue_preempt_only(q))) ||
			   blk_queue_dying(q));
		if (blk_queue_dying(q))
			return -ENODEV;
	}
}

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 Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.