linux-kernel - Re: [patch V3 48/64] locking/ww_mutex: Add RT priority to W/W order

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Message-ID: <20210806104835.GE22037@worktop.programming.kicks-ass.net>
Date:   Fri, 6 Aug 2021 12:48:35 +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>,
        Mike Galbraith <efault@....de>
Subject: Re: [patch V3 48/64] locking/ww_mutex: Add RT priority to W/W order

On Thu, Aug 05, 2021 at 05:13:48PM +0200, Thomas Gleixner wrote:
>  static inline bool
> +__ww_ctx_less(struct ww_acquire_ctx *a, struct ww_acquire_ctx *b)
>  {
> +/*
> + * Can only do the RT prio for WW_RT because task->prio isn't stable due to PI,
> + * so the wait_list ordering will go wobbly. rt_mutex re-queues the waiter and
> + * isn't affected by this.
> + */
> +#ifdef WW_RT
> +	/* kernel prio; less is more */
> +	int a_prio = a->task->prio;
> +	int b_prio = b->task->prio;
> +
> +	if (dl_prio(a_prio) || dl_prio(b_prio)) {

Whoever wrote this was an idiot :-) Both should be rt_prio().

> +
> +		if (a_prio > b_prio)
> +			return true;
> +
> +		if (a_prio < b_prio)
> +			return false;
> +
> +		/* equal static prio */
> +
> +		if (dl_prio(a_prio)) {
> +			if (dl_time_before(b->task->dl.deadline,
> +					   a->task->dl.deadline))
> +				return true;
> +
> +			if (dl_time_before(a->task->dl.deadline,
> +					   b->task->dl.deadline))
> +				return false;
> +		}
> +
> +		/* equal prio */
> +	}
> +#endif
>  
> +	/* FIFO order tie break -- bigger is younger */
>  	return (signed long)(a->stamp - b->stamp) > 0;
>  }