static inline void pipelined_send(struct wake_q_head *wake_q,
				  struct mqueue_inode_info *info,
				  struct msg_msg *message,
				  struct ext_wait_queue *receiver)
{
	receiver->msg = message;
	list_del(&receiver->list);
	wake_q_add(wake_q, receiver->task);
	/*
	 * Rely on the implicit cmpxchg barrier from wake_q_add such
	 * that we can ensure that updating receiver->state is the last
	 * write operation: As once set, the receiver can continue,
	 * and if we don't have the reference count from the wake_q,
	 * yet, at that point we can later have a use-after-free
	 * condition and bogus wakeup.
	 */
	receiver->state = STATE_READY;
}

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 Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.