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

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Date:   Mon, 02 Aug 2021 16:34:36 +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 Mon, Aug 02 2021 at 14:56, Peter Zijlstra wrote:
> On Fri, Jul 30, 2021 at 03:51:05PM +0200, Thomas Gleixner wrote:
> I did:
>
>  - atomic_cmpxchg() -> atomic_try_cmpxchg()

Ack.

>  - atomic_read() -> atomic_read_acquire(); which I think is required for
>    at least the futex_requeue_pi_wakeup_sync() >= Q_REQUEUE_PI_DONE case
>    to ensure we observe the state as per whoever set DONE/LOCKED.

Indeed

>  - use atomic_cond_read_relaxed()

Cute

>  - removed one ternary operator for (IMO) clearer code.

It's definitely more readable this way.

Thanks,

        tglx