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

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Date:   Tue, 03 Aug 2021 23:10:49 +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>
Subject: Re: [patch 58/63] futex: Prevent requeue_pi() lock nesting issue on RT

On Tue, Aug 03 2021 at 12:07, Peter Zijlstra wrote:

> 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.

Sure.