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

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Date:   Tue, 3 Aug 2021 13:20:51 +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 Fri, Jul 30, 2021 at 03:51:05PM +0200, Thomas Gleixner wrote:
> @@ -3082,27 +3302,22 @@ static int futex_unlock_pi(u32 __user *u
>  }
>  
>  /**
> - * handle_early_requeue_pi_wakeup() - Detect early wakeup on the initial futex
> + * handle_early_requeue_pi_wakeup() - Handle early wakeup on the initial futex
>   * @hb:		the hash_bucket futex_q was original enqueued on
>   * @q:		the futex_q woken while waiting to be requeued
> - * @key2:	the futex_key of the requeue target futex
>   * @timeout:	the timeout associated with the wait (NULL if none)
>   *
> - * Detect if the task was woken on the initial futex as opposed to the requeue
> - * target futex.  If so, determine if it was a timeout or a signal that caused
> - * the wakeup and return the appropriate error code to the caller.  Must be
> - * called with the hb lock held.
> + * Determine the cause for the early wakeup.
>   *
>   * Return:
> - *  -  0 = no early wakeup detected;
> - *  - <0 = -ETIMEDOUT or -ERESTARTNOINTR
> + *  -EWOULDBLOCK or -ETIMEDOUT or -ERESTARTNOINTR
>   */
>  static inline
>  int handle_early_requeue_pi_wakeup(struct futex_hash_bucket *hb,
> -				   struct futex_q *q, union futex_key *key2,
> +				   struct futex_q *q,
>  				   struct hrtimer_sleeper *timeout)
>  {
> -	int ret = 0;
> +	int ret;
>  
>  	/*
>  	 * With the hb lock held, we avoid races while we process the wakeup.
> @@ -3111,22 +3326,21 @@ int handle_early_requeue_pi_wakeup(struc
>  	 * It can't be requeued from uaddr2 to something else since we don't
>  	 * support a PI aware source futex for requeue.
>  	 */
> -	if (!match_futex(&q->key, key2)) {
> -		WARN_ON(q->lock_ptr && (&hb->lock != q->lock_ptr));
> -		/*
> -		 * We were woken prior to requeue by a timeout or a signal.
> -		 * Unqueue the futex_q and determine which it was.
> -		 */
> -		plist_del(&q->list, &hb->chain);
> -		hb_waiters_dec(hb);
> +	WARN_ON_ONCE(&hb->lock != q->lock_ptr);
>  
> -		/* Handle spurious wakeups gracefully */
> -		ret = -EWOULDBLOCK;
> -		if (timeout && !timeout->task)
> -			ret = -ETIMEDOUT;
> -		else if (signal_pending(current))
> -			ret = -ERESTARTNOINTR;
> -	}
> +	/*
> +	 * We were woken prior to requeue by a timeout or a signal.
> +	 * Unqueue the futex_q and determine which it was.
> +	 */
> +	plist_del(&q->list, &hb->chain);
> +	hb_waiters_dec(hb);
> +
> +	/* Handle spurious wakeups gracefully */
> +	ret = -EWOULDBLOCK;
> +	if (timeout && !timeout->task)
> +		ret = -ETIMEDOUT;
> +	else if (signal_pending(current))
> +		ret = -ERESTARTNOINTR;
>  	return ret;
>  }

AFAICT this change is a separate cleanup, possible because the only
callsite already does that match_futex() test before calling this.

I think it might be worth splitting out, just to reduce the complexity
of this patch.