Re: [patch 58/63] futex: Prevent requeue_pi() lock nesting issue on RT

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Aug 02, 2021 at 02:56:09PM +0200, Peter Zijlstra wrote:
>  static inline void futex_requeue_pi_complete(struct futex_q *q, int locked)
>  {
> +	int old, new;
>  
> +	old = atomic_read_acquire(&q->requeue_state);
> +	do {

		if (old == Q_REQUEUE_PI_IGNORE)
			return;

>  		if (locked >= 0) {

			WARN_ON_ONCE(old != Q_REQUEUE_PI_IN_PROGRESS &&
			             old != Q_REQUEUE_PI_WAIT)

>  			/* Requeue succeeded. Set DONE or LOCKED */
>  			new = Q_REQUEUE_PI_DONE + locked;
> +		} else if (old == Q_REQUEUE_PI_IN_PROGRESS) {
>  			/* Deadlock, no early wakeup interleave */
>  			new = Q_REQUEUE_PI_NONE;
>  		} else {

			WARN_ON_ONCE(old != Q_REQUEUE_PI_WAIT);

>  			/* Deadlock, early wakeup interleave. */
>  			new = Q_REQUEUE_PI_IGNORE;
>  		}
> +	} while (!atomic_try_cmpxchg(&q->requeue_state, &old, new));
>  
>  #ifdef CONFIG_PREEMPT_RT
>  	/* If the waiter interleaved with the requeue let it know */
> +	if (unlikely(old == Q_REQUEUE_PI_WAIT))
>  		rcuwait_wake_up(&q->requeue_wait);
>  #endif
>  }

I think...