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

[Peter Zijlstra]

On Mon, Aug 02, 2021 at 02:56:09PM +0200, Peter Zijlstra wrote:
>  static inline int futex_requeue_pi_wakeup_sync(struct futex_q *q)
>  {
> +	int old, new;
>  
> +	old = atomic_read_acquire(&q->requeue_state);
> +	do {
>  		/* Is requeue done already? */
> +		if (old >= Q_REQUEUE_PI_DONE)
>  			break;

I think that can be: return old; for slightly simpler code.

>  
>  		/*
>  		 * If not done, then tell the requeue code to either ignore
>  		 * the waiter or to wake it up once the requeue is done.
>  		 */
> +		new = Q_REQUEUE_PI_WAIT;
> +		if (old == Q_REQUEUE_PI_NONE)
> +			new = Q_REQUEUE_PI_IGNORE;
> +	} while (!atomic_try_cmpxchg(&q->requeue_state, &old, new));
>  
>  	/* If the requeue was in progress, wait for it to complete */
> +	if (old == Q_REQUEUE_PI_IN_PROGRESS) {
>  #ifdef CONFIG_PREEMPT_RT
>  		rcuwait_wait_event(&q->requeue_wait,
>  				   atomic_read(&q->requeue_state) != Q_REQUEUE_PI_WAIT,
>  				   TASK_UNINTERRUPTIBLE);
>  #else
> +		(void)atomic_cond_read_relaxed(&q->requeue_state, VAL != Q_REQUEUE_PI_WAIT);
>  #endif
>  	}
>