linux-kernel - Re: [PATCH v3 7/7] locking/rtmutex: Acquire the hb lock via trylock after wait-proxylock.

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Date: Tue, 16 Jan 2024 08:07:19 +0100
From: Jiri Slaby <jirislaby@...nel.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Peter Zijlstra <peterz@...radead.org>,
 Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
 boqun.feng@...il.com, bristot@...hat.com, bsegall@...gle.com,
 dietmar.eggemann@....com, jstultz@...gle.com, juri.lelli@...hat.com,
 longman@...hat.com, mgorman@...e.de, mingo@...hat.com, rostedt@...dmis.org,
 swood@...hat.com, vincent.guittot@...aro.org, vschneid@...hat.com,
 will@...nel.org
Subject: Re: [PATCH v3 7/7] locking/rtmutex: Acquire the hb lock via trylock
 after wait-proxylock.

On 15. 01. 24, 19:33, Sebastian Andrzej Siewior wrote:
> This duct tape at the end waits until the pi-state leaves or we get a
> waiter. So this works but is not a fix.

FWIW, it indeed avoids the problem.

> --- a/kernel/futex/pi.c
> +++ b/kernel/futex/pi.c
> @@ -1182,6 +1182,9 @@ int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)
>   		rt_waiter = rt_mutex_top_waiter(&pi_state->pi_mutex);
>   		if (!rt_waiter) {
>   			raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
> +			spin_unlock(&hb->lock);
> +			cpu_relax();
> +			goto retry;
>   			goto do_uncontended;
>   		}

thanks,
-- 
js
suse labs