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From:   Thomas Gleixner <tglx@linutronix.de>
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To:     Peter Zijlstra <peterz@infradead.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Will Deacon <will@kernel.org>,
        Waiman Long <longman@redhat.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Mike Galbraith <efault@gmx.de>
Subject: Re: [patch V3 50/64] locking/rtmutex: Extend the rtmutex core to
 support ww_mutex
In-Reply-To: <20210806110051.GF22037@worktop.programming.kicks-ass.net>
References: <20210805151300.330412127@linutronix.de>
 <20210805153955.709189588@linutronix.de>
 <20210806110051.GF22037@worktop.programming.kicks-ass.net>
Date:   Fri, 06 Aug 2021 13:19:15 +0200
Message-ID: <87fsvm6css.ffs@tglx>
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On Fri, Aug 06 2021 at 13:00, Peter Zijlstra wrote:

> On Thu, Aug 05, 2021 at 05:13:50PM +0200, Thomas Gleixner wrote:
>>  static __always_inline bool __waiter_less(struct rb_node *a, const struct rb_node *b)
>>  {
>> -	return rt_mutex_waiter_less(__node_2_waiter(a), __node_2_waiter(b));
>
> Given ^
>
>> +	struct rt_mutex_waiter *aw = __node_2_waiter(a);
>> +	struct rt_mutex_waiter *bw = __node_2_waiter(b);
>> +
>> +	if (rt_mutex_waiter_less(aw, bw))
>> +		return 1;
>
> We can, with this new build_ww_mutex(), do:
>
> 	if (!build_ww_mutex())
> 		return 0;
>
> here, to preserve the old behaviour.
>
>> +	if (rt_mutex_waiter_less(bw, aw))
>> +		return 0;
>> +
>> +	/* NOTE: relies on waiter->ww_ctx being set before insertion */
>> +	if (build_ww_mutex() && aw->ww_ctx) {
>
> Then it can go away here.

Pretty obvious. Indeed.