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

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Date:   Fri, 06 Aug 2021 13:50:37 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
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 Fri, Aug 06 2021 at 12:48, Peter Zijlstra wrote:

> 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().

Along with the idiot who picked that up and stared at it for quite some
time.