linux-kernel - Re: [PATCH 0/6] locking/rtmutex: Avoid PI state recursion through sched_submit

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Date:   Wed, 16 Aug 2023 17:25:11 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org,
        bsegall@...gle.com, boqun.feng@...il.com, swood@...hat.com,
        bristot@...hat.com, dietmar.eggemann@....com, mingo@...hat.com,
        jstultz@...gle.com, juri.lelli@...hat.com, mgorman@...e.de,
        rostedt@...dmis.org, vschneid@...hat.com,
        vincent.guittot@...aro.org, longman@...hat.com, will@...nel.org
Subject: Re: [PATCH 0/6] locking/rtmutex: Avoid PI state recursion through
 sched_submit_work()

On 2023-08-16 17:22:31 [+0200], Peter Zijlstra wrote:
> On Wed, Aug 16, 2023 at 04:58:18PM +0200, Peter Zijlstra wrote:
> 
> > I've ended up with the below, but it is quite horrible.. but let me go
> > stare at the futex wreckage before trying to clean things up.
> 
> OK, I think the below covers the simple case, now lets see if I can make
> sense of futex_wait_requeue_pi()... :/
> 
> ---
> --- a/kernel/futex/pi.c
> +++ b/kernel/futex/pi.c
> @@ -1002,6 +1002,12 @@ int futex_lock_pi(u32 __user *uaddr, uns
>  		goto no_block;
>  	}
>  
> +	/*
> +	 * Must be done before we enqueue the waiter, here is unfortunately
> +	 * under the hb lock, but that *should* work.
> +	 */
> +	rt_mutex_pre_schedule();

but this will do sched_submit_work() which you don't need for futex at
all. It should be a nop (as in nothing will happen) but still.

>  	rt_mutex_init_waiter(&rt_waiter);
>  
>  	/*

Sebastian