linux-kernel - Re: [patch 58/63] futex: Prevent requeue

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Date:   Mon, 2 Aug 2021 15:10:06 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [patch 58/63] futex: Prevent requeue_pi() lock nesting issue on
 RT

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
>  	}
>