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

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Message-ID: <20210803100713.GB8057@worktop.programming.kicks-ass.net>
Date:   Tue, 3 Aug 2021 12:07:13 +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:
> @@ -219,6 +221,10 @@ struct futex_q {
>  	struct rt_mutex_waiter *rt_waiter;
>  	union futex_key *requeue_pi_key;
>  	u32 bitset;
> +	atomic_t requeue_state;
> +#ifdef CONFIG_PREEMPT_RT
> +	struct rcuwait requeue_wait;
> +#endif
>  } __randomize_layout;
>  
>  static const struct futex_q futex_q_init = {

Do we want to explicitly initialize requeue_state in futex_q_init? I was
looking where we reset the state machine and eventually figured it out,
but I'm thinking something more explicit might help avoid this for the
next time.